Which seas and ferry crossings are roughest in May
Of the 54 seas and ferry routes on this site, 8 sit at rough or roughest in May and 27 are calm. 2 are at the hardest level they reach in any month of the year. Against April, 2 step up a level and 15 step down.
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Every water ranked for May
Each level below is the one that water carries for May 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 May: 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 May: 7 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 |
|---|---|---|
| Bass StraitSea area, Southern Australia | Ocean swell shortening over a 60 m shelf | high |
| 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 |
| 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 May: 19 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 |
| Bali (Padangbai) to Gili TrawanganFerry route, Asia | Deep strait currents under small fast boats | high |
| Bar Harbor to Yarmouth (The CAT)Ferry route, North America | Open Gulf of Maine at catamaran speed | 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 |
| Dar es Salaam to ZanzibarFerry route, Africa | Indian Ocean swell across an open channel | moderate |
| Gulf of AlaskaSea area, North Pacific | Aleutian Low storms, no island shelter | high |
| Gulf of LionSea area, Western Mediterranean | Mistral funnelled down the Rhône valley | high |
| Hirtshals to TorshavnFerry route, Northern Europe | A day and a half into the open North Atlantic | high |
| Hong Kong to MacauFerry route, Asia | An estuary that opens to the South China Sea | moderate |
| Hyannis to NantucketFerry route, North America | Shallow sound chop under fast catamarans | moderate |
| 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 |
| North SeaSea area, North-west Europe | Shallow basin, short steep wind chop | high |
| 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 |
| Scrabster to StromnessFerry route, UK and Ireland | Tidal races, not wind-driven waves | very high |
| Strait of GibraltarSea area, Spain and Morocco | Wind funneled between two mountain ranges | moderate |
Calm in May: 27 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 |
| Aegean SeaSea area, Eastern Mediterranean | Summer meltemi funnelled down the Aegean | moderate |
| Baltic SeaSea area, Northern Europe | Short steep chop, no ocean swell | moderate |
| Barcelona to Palma de MallorcaFerry route, Southern Europe | Tramontana and mistral chop on an open crossing | moderate |
| Busan to Hakata (Fukuoka)Ferry route, Asia | The overnight strait, after the fast craft era | moderate |
| Cairnryan to BelfastFerry route, UK and Ireland | Atlantic weather funnelled through the North Channel | moderate |
| Cape May to LewesFerry route, North America | Where Atlantic swell meets the estuary | 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 |
| Helsinki to TallinnFerry route, Northern Europe | A short hop on outsized ships | low |
| Holyhead to DublinFerry route, UK and Ireland | Wind against strong tidal streams | moderate |
| Irish SeaSea area, British Isles | Wind against strong tidal streams | moderate |
| Korea StraitSea area, East Asia | Monsoon chop over a shallow shelf | 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 |
| Naples to CapriFerry route, Southern Europe | Small fast hulls on an open bay approach | moderate |
| Oslo to KielFerry route, Northern Europe | Big ships, one open strait in the middle | moderate |
| 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 |
| Sea of JapanSea area, East Asia | Winter monsoon over a long open fetch | high |
| Seattle to Bainbridge IslandFerry route, North America | The sheltered sound's flagship commute | low |
| SkagerrakSea area, Scandinavia | North Sea swell funneled into a narrowing strait | moderate |
| Split to HvarFerry route, Southern Europe | Island-sheltered channel, bora-day exception | moderate |
| The MinchSea area, North-west Scotland | Atlantic swell down an open northern door | moderate |
| Vancouver (Tsawwassen) to Victoria (Swartz Bay)Ferry route, North America | Open strait first, island maze after | low |
How May compares with the months either side
Coming out of April, 2 of these waters move up a level and 15 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 May?
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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.