Which seas and ferry crossings are roughest in September
Of the 54 seas and ferry routes on this site, 14 sit at rough or roughest in September and 9 are calm. 7 are at the hardest level they reach in any month of the year. Against August, 21 step up a level and 10 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 54
This water at its hardest. These are the months its reputation comes from, and the ones passengers write about afterwards.
| 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: 13 of 54
The difficult half of this water's year. Calm days still happen, but bad ones are common enough to plan around.
| Sea or route | What drives it | Rest of its year |
|---|---|---|
| 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 |
| Busan to Hakata (Fukuoka)Ferry route, Asia. Worst level of its year. | The overnight strait, after the fast craft era | 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 |
| 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 |
| Tasman SeaSea area, South-west Pacific | 2,000 km of unbroken westerly fetch | high |
| Wellington to PictonFerry route, Australia and New Zealand | 75 exposed minutes in a wind funnel | high |
Mixed in September: 31 of 54
It depends on the week. Settled weather gives an easy passage here, an approaching front gives an uncomfortable one.
| 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 |
| 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 |
| 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 |
| Dar es Salaam to ZanzibarFerry route, Africa | Indian Ocean swell across an open channel | moderate |
| Dover to CalaisFerry route, UK and Ireland | Shallow strait, wind against tide | moderate |
| English ChannelSea area, North-west Europe | Shallow strait, wind against tide | moderate |
| Fort Myers Beach to Key West (Key West Express)Ferry route, North America | A long open-Gulf run at speed | moderate |
| Holyhead to DublinFerry route, UK and Ireland | Wind against strong tidal streams | moderate |
| Hyannis to NantucketFerry route, North America | Shallow sound chop under fast catamarans | moderate |
| Irish SeaSea area, British Isles | Wind against strong tidal streams | 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 |
| North SeaSea area, North-west Europe | Shallow basin, short steep wind chop | high |
| Oslo to KielFerry route, Northern Europe | Big ships, one open strait in the middle | 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 |
| Piraeus to Heraklion (Crete)Ferry route, Southern Europe | The full meltemi fetch, slept through on big ships | 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 |
| 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 |
| SkagerrakSea area, Scandinavia | North Sea swell funneled into a narrowing strait | moderate |
| Strait of GibraltarSea area, Spain and Morocco | Wind funneled between two mountain ranges | moderate |
| The MinchSea area, North-west Scotland | Atlantic swell down an open northern door | moderate |
Calm in September: 9 of 54
The settled end of this water's year, when most passages pass without much motion worth noticing.
| Sea or route | What drives it | Rest of its year |
|---|---|---|
| Adriatic SeaSea area, Central Mediterranean | Katabatic bora gusts off the Dinaric Alps | moderate |
| Gulf of LionSea area, Western Mediterranean | Mistral funnelled down the Rhône valley | high |
| Helsinki to TallinnFerry route, Northern Europe | A short hop on outsized ships | low |
| Long Beach to Catalina IslandFerry route, North America | Pacific ground swell under fast hulls | moderate |
| Mozambique ChannelSea area, South-west Indian Ocean | Cyclone corridor feeding the Agulhas Current | high |
| Naples to CapriFerry route, Southern Europe | Small fast hulls on an open bay approach | moderate |
| Seattle to Bainbridge IslandFerry route, North America | The sheltered sound's flagship commute | low |
| Split to HvarFerry route, Southern Europe | Island-sheltered channel, bora-day exception | moderate |
| Vancouver (Tsawwassen) to Victoria (Swartz Bay)Ferry route, North America | Open strait first, island maze after | low |
How September compares with the months either side
Coming out of August, 21 of these waters move up a level and 10 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.
Sailing somewhere on this list in September?
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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.