Which seas and ferry crossings are roughest in September

Of the 202 seas and ferry routes on this site, 29 sit at rough or roughest in September and 55 are calm. 19 are at the hardest level they reach in any month of the year. Against August, 80 step up a level and 29 step down.

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Every water ranked for September

Each level below is the one that water carries for September on its own page. It describes the typical pattern of the month, not a forecast for your sailing date. A water marked as its own worst is at the hardest level it reaches in any month of the year.

Roughest in September: 1 of 202

This water at its hardest. These are the months its reputation comes from, and the ones passengers write about afterwards.

Seas and ferry routes rated roughest in September
Sea or route	What drives it	Rest of its year
Drake PassageSea area, Southern Ocean. Worst level of its year.	No land to stop wind, swell or current	very high

Rough in September: 28 of 202

The difficult half of this water's year. Calm days still happen, but bad ones are common enough to plan around.

Seas and ferry routes rated rough in September
Sea or route	What drives it	Rest of its year
Andaman SeaSea area, Thailand and the Andaman Islands	Monsoon winds over deep open water	moderate
Backstairs PassageSea area, South Australia. Worst level of its year.	Narrow tidal channel, Southern Ocean wind	high
Bali (Padangbai) to Gili TrawanganFerry route, Asia. Worst level of its year.	Deep strait currents under small fast boats	high
Bass StraitSea area, Southern Australia	Ocean swell shortening over a 60 m shelf	high
Bering SeaSea area, Alaska	Aleutian Low storms over a wide deep basin	very high
Bluff to Oban (Stewart Island)Ferry route, Australia and New Zealand	Roaring Forties swell over a shallow seaway	very high
Busan to Hakata (Fukuoka)Ferry route, Asia. Worst level of its year.	The overnight strait, after the fast craft era	moderate
Busan to OsakaFerry route, Asia. Worst level of its year.	One open strait, then Japan's inland sea	moderate
Busan to ShimonosekiFerry route, Asia. Worst level of its year.	The historic overnight run across the strait	moderate
Caribbean SeaSea area, Caribbean. Worst level of its year.	Trade winds, winter surges and hurricane season	moderate
Cook StraitSea area, New Zealand	Wind against a 5-knot tidal stream	high
Foveaux StraitSea area, Southern New Zealand	Roaring Forties swell over a shallow seaway	very high
Geelong to Devonport (Spirit of Tasmania)Ferry route, Australia and New Zealand	Ocean swell shortening over a 60 m shelf	high
Great Australian BightSea area, Southern Australia	Southern Ocean swell with unlimited fetch	high
Gulf of AlaskaSea area, North Pacific	Aleutian Low storms, no island shelter	high
Gulf of MexicoSea area, North America. Worst level of its year.	Winter northers behind cold fronts	moderate
Hauraki GulfSea area, New Zealand. Worst level of its year.	Protected gulf, exposed to north winds	moderate
Hirtshals to TorshavnFerry route, Northern Europe	A day and a half into the open North Atlantic	high
Hong Kong to MacauFerry route, Asia. Worst level of its year.	An estuary that opens to the South China Sea	moderate
Korea StraitSea area, East Asia. Worst level of its year.	Monsoon chop over a shallow shelf	moderate
Lake SuperiorSea area, Great Lakes (Michigan, Minnesota, Wisconsin and Ontario)	Huge fetch over deep water	high
Moreton BaySea area, Brisbane, Australia. Worst level of its year.	Shallow bay behind sand islands, wind chop	moderate
Pearl River EstuarySea area, Southern China. Worst level of its year.	Shallow estuary, monsoon wind and typhoons	moderate
Phuket to Koh Phi PhiFerry route, Asia. Worst level of its year.	A 45 km open Andaman crossing with two seasons	moderate
Point Judith to Block IslandFerry route, North America. Worst level of its year.	A short hop open to Atlantic swell	moderate
Tasman SeaSea area, South-west Pacific	2,000 km of unbroken westerly fetch	high
Torshavn to SeydisfjordurFerry route, Northern Europe	High-latitude open ocean, the Iceland leg	high
Wellington to PictonFerry route, Australia and New Zealand	75 exposed minutes in a wind funnel	high

Mixed in September: 118 of 202

It depends on the week. Settled weather gives an easy passage here, an approaching front gives an uncomfortable one.

Seas and ferry routes rated mixed in September
Sea or route	What drives it	Rest of its year
Aberdeen to LerwickFerry route, UK and Ireland	216 miles up the open northern North Sea	high
Aegean SeaSea area, Eastern Mediterranean	Summer meltemi funnelled down the Aegean	moderate
Algeciras to CeutaFerry route, Southern Europe	The sheltered-side shortcut	moderate
Algeciras to Tanger MedFerry route, Southern Europe	The strait's big-ship crossing	moderate
Balearic SeaSea area, Spain	Short fetch, northerly storms and fronts	moderate
Baltic SeaSea area, Northern Europe	Short steep chop, no ocean swell	moderate
Bar Harbor to Yarmouth (The CAT)Ferry route, North America	Open Gulf of Maine at catamaran speed	moderate
Barcelona to Palma de MallorcaFerry route, Southern Europe	Tramontana and mistral chop on an open crossing	moderate
Bay of BiscaySea area, North-east Atlantic	Atlantic swell over a shoaling shelf	high
Bay of IslandsSea area, Northland, New Zealand	Open Pacific at Cape Brett, islands inside	moderate
Buenos Aires to Colonia del SacramentoFerry route, Caribbean and Latin America. Worst level of its year.	A shallow brown-water estuary with one bad wind	low
Cabot StraitSea area, Atlantic Canada	Atlantic storm track across an open gulf mouth	high
Cairnryan to BelfastFerry route, UK and Ireland	Atlantic weather funnelled through the North Channel	moderate
Cape May to LewesFerry route, North America. Worst level of its year.	Where Atlantic swell meets the estuary	moderate
Cebu StraitSea area, Philippines	Island channel, rougher in monsoons and typhoons	moderate
Chumphon to Koh TaoFerry route, Asia	Gulf chop on the opposite monsoon calendar	moderate
Copenhagen to OsloFerry route, Northern Europe	Sheltered start, open Skagerrak in the night	moderate
Coral SeaSea area, South-west Pacific	Trade-wind chop behind a reef screen	moderate
Dar es Salaam to ZanzibarFerry route, Africa	Indian Ocean swell across an open channel	moderate
Delaware BaySea area, New Jersey and Delaware	Open bay, long fetch and strong currents	moderate
Denia to IbizaFerry route, Southern Europe	Fast catamarans on an open Balearic hop	moderate
Dover to CalaisFerry route, UK and Ireland	Shallow strait, wind against tide	moderate
Dover to DunkirkFerry route, UK and Ireland	Shallow strait, wind against tide	moderate
East China SeaSea area, Southern Japan	Typhoons and fronts over a shallow sea	high
English ChannelSea area, North-west Europe	Shallow strait, wind against tide	moderate
Fishguard to RosslareFerry route, UK and Ireland	Strong tides meeting open Celtic Sea swell	moderate
Fort Lauderdale to BiminiFerry route, North America	Wind against the Gulf Stream's current	moderate
Fort Myers Beach to Key West (Key West Express)Ferry route, North America	A long open-Gulf run at speed	moderate
Galway BaySea area, West of Ireland	Atlantic swell entering through the Aran sounds	moderate
German BightSea area, North Sea coast of Germany	Shallow open bight with tidal channels	high
Gills Bay to St Margaret's HopeFerry route, UK and Ireland	Straight across the world's fastest tidal streams	very high
Gulf of CaliforniaSea area, Mexico	Long, deep gulf, brief chubasco storms	moderate
Gulf of FinlandSea area, Finland and Estonia	Narrow gulf, wind along its axis	moderate
Gulf of PenasSea area, Chilean Patagonia	Open Pacific, westerly storms	very high
Gulf of SalernoSea area, Amalfi Coast, Italy	Wide open Tyrrhenian mouth, winter storms	moderate
Gulf of St. LawrenceSea area, Eastern Canada	Open gulf, autumn wind waves, winter ice	high
Gulf of ThailandSea area, South-east Asia	Shallow gulf, monsoon wind chop	moderate
Ha Long BaySea area, Northern Vietnam. Worst level of its year.	Island-shielded bay, storm-season closures	moderate
Harwich to Hook of HollandFerry route, UK and Ireland	The shallowest water on the North Sea ferry map	high
Hecate StraitSea area, British Columbia	Shallow strait, steep wind waves, southeast gales	very high
Heysham to DouglasFerry route, UK and Ireland	The year-round workhorse across open water	moderate
Hirtshals to KristiansandFerry route, Northern Europe	North Sea swell funnelled into the Skagerrak	moderate
Hirtshals to LarvikFerry route, Northern Europe	The longer diagonal across the open Skagerrak	moderate
Holyhead to DublinFerry route, UK and Ireland	Wind against strong tidal streams	moderate
Hull to RotterdamFerry route, UK and Ireland	Overnight across the shallow southern basin	high
Hyannis to NantucketFerry route, North America	Shallow sound chop under fast catamarans	moderate
Inside PassageSea area, North Pacific. Worst level of its year.	Island shelter, except at its open crossings	moderate
Irish SeaSea area, British Isles	Wind against strong tidal streams	moderate
Jeju StraitSea area, South Korea	Winter wind over a strait between two seas	high
KattegatSea area, Denmark and Sweden	Shallow sea with tiny tides	moderate
Lake HuronSea area, Great Lakes (Ontario and Michigan)	Open fetch, winds from any quarter	high
Lake MichiganSea area, US Midwest	Long fetch under autumn gales	high
Ligurian SeaSea area, North-west Italy and Corsica	Mistral and Gulf of Genoa lows over deep water	moderate
Liverpool to BelfastFerry route, UK and Ireland	The long diagonal of the Irish Sea	moderate
Liverpool to DouglasFerry route, UK and Ireland	Open Irish Sea, two very different hulls	moderate
Maizuru to OtaruFerry route, Asia	A day at sea along the winter monsoon's fetch	high
Mediterranean SeaSea area, Mediterranean	Mistral in the west, meltemi in the Aegean	moderate
Mona PassageSea area, Caribbean	Tidal current forced over shallow banks	moderate
Mykonos to SantoriniFerry route, Southern Europe	Beam-on to the meltemi in small fast craft	high
Newcastle to AmsterdamFerry route, UK and Ireland	A full night across the open middle	high
Newhaven to DieppeFerry route, UK and Ireland	Open mid-Channel, longer in the swell	moderate
Niigata to OtaruFerry route, Asia	The shorter Hokkaido run, same winter sea	high
Niigata to Ryotsu (Sado Island)Ferry route, Asia	A short hop open to the winter sea	high
North SeaSea area, North-west Europe	Shallow basin, short steep wind chop	high
North Sydney to ArgentiaFerry route, North America	The long way to Newfoundland, more Atlantic in it	high
North Sydney to Port aux BasquesFerry route, North America	The Atlantic storm track's ferry crossing	high
Norwegian SeaSea area, Northern Europe	Ocean swell at unsheltered capes	high
Oslo to KielFerry route, Northern Europe	Big ships, one open strait in the middle	moderate
Pembroke to RosslareFerry route, UK and Ireland	The longer diagonal across St George's Channel	moderate
Pentland FirthSea area, Northern Scotland	Tidal races, not wind-driven waves	very high
Penzance to St Mary's (Isles of Scilly)Ferry route, UK and Ireland	Open Atlantic swell met by a flat-bottomed hull	high
Pillsbury SoundSea area, US Virgin Islands	Trade-wind chop in a small sound	moderate
Piraeus to Heraklion (Crete)Ferry route, Southern Europe	The full meltemi fetch, slept through on big ships	moderate
Piraeus to MykonosFerry route, Southern Europe	Open central Aegean in the meltemi's lane	moderate
Piraeus to SantoriniFerry route, Southern Europe	The long run down the meltemi's fetch	moderate
Playa del Carmen to CozumelFerry route, Caribbean and Latin America	Wind against the Yucatan current	moderate
Plymouth to RoscoffFerry route, UK and Ireland	The Channel's open Atlantic doorway	moderate
Plymouth to SantanderFerry route, UK and Ireland	The short cut that still crosses all of Biscay	high
Poole to CherbourgFerry route, UK and Ireland	Open mid-Channel on the beam	moderate
Port Angeles to Victoria (MV Coho)Ferry route, North America	Straight across the Pacific's doorway	moderate
Portsmouth to BilbaoFerry route, UK and Ireland	A long offshore middle over open Biscay	high
Portsmouth to CaenFerry route, UK and Ireland	Long open leg across the central Channel	moderate
Portsmouth to SantanderFerry route, UK and Ireland	Atlantic swell over a shoaling shelf	high
Portsmouth to St MaloFerry route, UK and Ireland	The longest Channel crossing, slept through	moderate
Prince William SoundSea area, South-central Alaska	Sheltered fjords, Gulf swell at the entrance	moderate
Resurrection BaySea area, Southcentral Alaska	Sheltered fjord, open Gulf of Alaska swell beyond	moderate
Rosslare to CherbourgFerry route, UK and Ireland	Open Celtic Sea with full Atlantic fetch	high
Rottnest ChannelSea area, Western Australia	Open-shelf swell, summer sea breeze	moderate
San Juan to Santo DomingoFerry route, Caribbean and Latin America	The Caribbean's trickiest passage, overnight	moderate
Saronic GulfSea area, Athens, Greece	Summer meltemi, partly sheltered by Attica	moderate
Scrabster to StromnessFerry route, UK and Ireland	Tidal races, not wind-driven waves	very high
Sea of JapanSea area, East Asia	Winter monsoon over a long open fetch	high
Sea of the HebridesSea area, West coast of Scotland	Partly sheltered sea with Atlantic storms in winter	high
Seattle to Victoria (Victoria Clipper)Ferry route, North America	Two sheltered hours, one open strait	moderate
Seto Inland SeaSea area, Western Japan	Sheltered inland sea, strong tidal straits	moderate
Singapore StraitSea area, South-east Asia	Sheltered strait, monsoon wind chop	moderate
Sir Francis Drake ChannelSea area, British Virgin Islands	Steady trade winds between islands	moderate
SkagerrakSea area, Scandinavia	North Sea swell funneled into a narrowing strait	moderate
Stockholm to HelsinkiFerry route, Northern Europe	Archipelago shelter, one open Baltic gap	moderate
Stockholm to TallinnFerry route, Northern Europe	The open Baltic Proper in the middle of the night	moderate
Strait of BonifacioSea area, Corsica and Sardinia	Mistral squeezed through an 11 km gap	high
Strait of GibraltarSea area, Spain and Morocco	Wind funneled between two mountain ranges	moderate
Strait of MagellanSea area, Southern Chile	Narrows, strong tide and wind	high
Strait of MalaccaSea area, Malaysia and Indonesia	Sheltered strait with Sumatra squalls	moderate
Strait of MessinaSea area, Sicily and Calabria	Opposed tides drive strong currents through a 3 km gap	moderate
Straits of MackinacSea area, Northern Michigan	Irregular currents and autumn gales between two lakes	moderate
Sydney HarbourSea area, New South Wales, Australia	Harbour with an open ocean entrance	moderate
Table BaySea area, South Africa	Open to the north-west, winter storm waves	moderate
Tarifa to TangierFerry route, Southern Europe	Wind funnelled between two continents	moderate
The MinchSea area, North-west Scotland	Atlantic swell down an open northern door	moderate
Uig to Lochmaddy (North Uist)Ferry route, UK and Ireland	The Uist leg of the Little Minch triangle	moderate
Uig to Tarbert (Harris)Ferry route, UK and Ireland	The Little Minch's open crossing to Harris	moderate
Ullapool to StornowayFerry route, UK and Ireland	Atlantic swell down an open northern door	moderate
Valletta to Pozzallo (Malta to Sicily)Ferry route, Southern Europe	An open channel crossed at 38 knots	moderate
Verde Island PassageSea area, Philippines	Two monsoons and a typhoon season	moderate
VestfjordenSea area, Northern Norway	Open bight of sea with strong tidal eddies	high
Vieques SoundSea area, Puerto Rico	Steady trade winds over a long fetch, shoals in the west	moderate
Zanzibar ChannelSea area, East Africa	Open Indian Ocean channel, monsoon winds	moderate

Calm in September: 55 of 202

The settled end of this water's year, when most passages pass without much motion worth noticing.

Seas and ferry routes rated calm in September
Sea or route	What drives it	Rest of its year
Adriatic SeaSea area, Central Mediterranean	Katabatic bora gusts off the Dinaric Alps	moderate
Alboran SeaSea area, Spain and Morocco	Open water, roughest from winter to spring	moderate
Ancona to IgoumenitsaFerry route, Southern Europe	Running the whole Adriatic, north to south	moderate
Ancona to SplitFerry route, Southern Europe	Overnight across the open central Adriatic	moderate
Auckland to Waiheke IslandFerry route, Australia and New Zealand	An island-sheltered gulf commute	low
Barents SeaSea area, Arctic Norway	Open Arctic water, winter polar lows	high
Bari to DubrovnikFerry route, Southern Europe	The deep southern Adriatic basin	moderate
Bay of FundySea area, New Brunswick and Nova Scotia	Record tides, swell against the ebb at Digby Gut	moderate
Bay of NaplesSea area, Southern Italy	West-facing bay, autumn and winter storms	moderate
Block Island SoundSea area, Rhode Island, Connecticut and New York	Open to the Atlantic, southerly fetch	moderate
Bristol ChannelSea area, South-west England and Wales	Atlantic swell and a 13 m tidal range	high
Ceiba to Vieques and CulebraFerry route, Caribbean and Latin America	Trade-wind chop across an island sound	moderate
Celtic SeaSea area, Between Ireland, Cornwall and Brittany	Open Atlantic swell over a shelf sea	high
Civitavecchia to OlbiaFerry route, Southern Europe	Open Tyrrhenian in the mistral's throw	moderate
Flores SeaSea area, Eastern Indonesia	Fast currents in the island channels	moderate
Gulf of LionSea area, Western Mediterranean	Mistral funnelled down the Rhône valley	high
Gulf of MaineSea area, Maine and Nova Scotia	Open Atlantic gulf, winter gales on a long fetch	high
Gulf of Saint-MaloSea area, Between Brittany, Normandy and the Channel Islands	Huge tides and island reefs in the western Channel	moderate
Helsinki to TallinnFerry route, Northern Europe	A short hop on outsized ships	low
Ionian SeaSea area, Western Greece and southern Italy	Winter southerlies over deep open water	moderate
Lake ErieSea area, Great Lakes (Ohio and Ontario)	Shallow lake, seas build fast	high
Long Beach to Catalina IslandFerry route, North America	Pacific ground swell under fast hulls	moderate
Long Island SoundSea area, New York and Connecticut	Sheltered sound, long east-west fetch	moderate
Malta ChannelSea area, Central Mediterranean	Open channel exposed to north-westerly gales	moderate
Marseille to AjaccioFerry route, Southern Europe	Mistral funnelled down the Rhone valley	high
Monterey BaySea area, Central California	Winter storms and swell in an open bight	moderate
Mozambique ChannelSea area, South-west Indian Ocean	Cyclone corridor feeding the Agulhas Current	high
Nantucket SoundSea area, Cape Cod, Massachusetts	Winter gales over shoal-filled water	moderate
Naples to CapriFerry route, Southern Europe	Small fast hulls on an open bay approach	moderate
Naples to PalermoFerry route, Southern Europe	A full-length open Tyrrhenian night	moderate
Nice to BastiaFerry route, Southern Europe	Open Ligurian Sea on fast hulls	moderate
Puttgarden to RodbyFerry route, Northern Europe	A 45 minute belt hop with land in sight	low
Saint John to Digby (Fundy Rose)Ferry route, North America	The world's biggest tides, mostly behaving	moderate
Salish SeaSea area, North Pacific	Mountain-sheltered basins, one strait open to the Pacific	moderate
San Francisco BaySea area, Northern California	Funneled summer sea breeze and tidal swirls	moderate
San Pedro ChannelSea area, Southern California	Open Pacific channel, spring peak	moderate
Santa Barbara ChannelSea area, Southern California	Afternoon northwest wind along the islands' north shores	high
Sea of ÅlandSea area, Sweden and the Åland Islands	Deep basin joined to the open Baltic	moderate
Seattle to Bainbridge IslandFerry route, North America	The sheltered sound's flagship commute	low
South China SeaSea area, Southeast Asia	Northeast monsoon wind over a deep, open sea	high
Split to HvarFerry route, Southern Europe	Island-sheltered channel, bora-day exception	moderate
St George's ChannelSea area, Wales and Ireland	Channel linking the Celtic Sea and the Irish Sea	high
St Lucia ChannelSea area, Eastern Caribbean (Martinique and Saint Lucia)	Trade-wind sea in a gap between islands	moderate
St Lucia to Martinique (Castries to Fort-de-France)Ferry route, Caribbean and Latin America	Trade swell funnelling between the islands	moderate
Taiwan StraitSea area, Taiwan	Northeast monsoon over a shallow strait	high
Toulon to BastiaFerry route, Southern Europe	Mistral seas from the gulf's eastern edge	high
Trinidad to Tobago (Port of Spain to Scarborough)Ferry route, Caribbean and Latin America	Calm gulf, open Atlantic middle	high
Tsugaru StraitSea area, Japan	Winter monsoon seas into a 20 km gap	high
Turku to StockholmFerry route, Northern Europe	One of Europe's calmest long ferry rides	low
Tyrrhenian SeaSea area, Western Italy, Sardinia and Sicily	Winter mistral and libeccio over deep water	moderate
Vancouver (Tsawwassen) to Victoria (Swartz Bay)Ferry route, North America	Open strait first, island maze after	low
Vineyard SoundSea area, Cape Cod, Massachusetts	Wind against tidal current at West Chop	moderate
Wadden SeaSea area, Netherlands, Germany and Denmark	Sheltered tidal flats, wind-driven chop	moderate
Whitsunday PassageSea area, Queensland, Australia	Wind across the gap to the islands	moderate
Woods Hole to Martha's VineyardFerry route, North America	The shorter, more sheltered island run	low

How September compares with the months either side

Coming out of August, 80 of these waters move up a level and 29 move down. The rest hold where they were. Roughness does not switch on in one month: most of these waters shift a step at a time, which is why the shoulder months either side of a bad season are often the useful booking window.

The two hemispheres also run opposite. A month at the calm end of the year in the North Atlantic is the hard end of it in the Southern Ocean, so a single month page holds both a list of waters to avoid and a list to book.

Previous month

Roughest water in August

Next month

Roughest water in October

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Levels on this page are the typical seasonal patterns published on each sea and route page, drawn from marine and operator information as of September 2026. They are not forecasts. Route and operator names are trademarks of their respective owners; Dizzout is not affiliated with or endorsed by them. This page is informational and not medical advice.