Which seas and ferry crossings are roughest in December
Of the 54 seas and ferry routes on this site, 34 sit at rough or roughest in December and one is calm. 38 are at the hardest level they reach in any month of the year. Against November, 11 step up a level and 5 step down.
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Every water ranked for December
Each level below is the one that water carries for December 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 December: 9 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 |
|---|---|---|
| Aberdeen to LerwickFerry route, UK and Ireland. Worst level of its year. | 216 miles up the open northern North Sea | high |
| Bay of BiscaySea area, North-east Atlantic. Worst level of its year. | Atlantic swell over a shoaling shelf | high |
| Cabot StraitSea area, Atlantic Canada. Worst level of its year. | Atlantic storm track across an open gulf mouth | high |
| Gulf of AlaskaSea area, North Pacific. Worst level of its year. | Aleutian Low storms, no island shelter | high |
| Hirtshals to TorshavnFerry route, Northern Europe. Worst level of its year. | A day and a half into the open North Atlantic | high |
| North SeaSea area, North-west Europe. Worst level of its year. | Shallow basin, short steep wind chop | high |
| Pentland FirthSea area, Northern Scotland. Worst level of its year. | Tidal races, not wind-driven waves | very high |
| Scrabster to StromnessFerry route, UK and Ireland. Worst level of its year. | Tidal races, not wind-driven waves | very high |
| Sea of JapanSea area, East Asia. Worst level of its year. | Winter monsoon over a long open fetch | high |
Rough in December: 25 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 |
|---|---|---|
| Adriatic SeaSea area, Central Mediterranean. Worst level of its year. | Katabatic bora gusts off the Dinaric Alps | moderate |
| Baltic SeaSea area, Northern Europe. Worst level of its year. | Short steep chop, no ocean swell | moderate |
| Bar Harbor to Yarmouth (The CAT)Ferry route, North America. Worst level of its year. | Open Gulf of Maine at catamaran speed | moderate |
| Barcelona to Palma de MallorcaFerry route, Southern Europe. Worst level of its year. | Tramontana and mistral chop on an open crossing | moderate |
| Busan to Hakata (Fukuoka)Ferry route, Asia. Worst level of its year. | The overnight strait, after the fast craft era | moderate |
| Cairnryan to BelfastFerry route, UK and Ireland. Worst level of its year. | Atlantic weather funnelled through the North Channel | moderate |
| Dover to CalaisFerry route, UK and Ireland. Worst level of its year. | Shallow strait, wind against tide | moderate |
| Drake PassageSea area, Southern Ocean | No land to stop wind, swell or current | very high |
| English ChannelSea area, North-west Europe. Worst level of its year. | Shallow strait, wind against tide | moderate |
| Fort Myers Beach to Key West (Key West Express)Ferry route, North America. Worst level of its year. | A long open-Gulf run at speed | moderate |
| Gulf of LionSea area, Western Mediterranean. Worst level of its year. | Mistral funnelled down the Rhône valley | high |
| Holyhead to DublinFerry route, UK and Ireland. Worst level of its year. | Wind against strong tidal streams | moderate |
| Hyannis to NantucketFerry route, North America. Worst level of its year. | Shallow sound chop under fast catamarans | moderate |
| Irish SeaSea area, British Isles. Worst level of its year. | Wind against strong tidal streams | moderate |
| Korea StraitSea area, East Asia. Worst level of its year. | Monsoon chop over a shallow shelf | moderate |
| Mona PassageSea area, Caribbean. Worst level of its year. | Tidal current forced over shallow banks | moderate |
| Mozambique ChannelSea area, South-west Indian Ocean. Worst level of its year. | Cyclone corridor feeding the Agulhas Current | high |
| Oslo to KielFerry route, Northern Europe. Worst level of its year. | Big ships, one open strait in the middle | moderate |
| Penzance to St Mary's (Isles of Scilly)Ferry route, UK and Ireland. Worst level of its year. | Open Atlantic swell met by a flat-bottomed hull | high |
| Playa del Carmen to CozumelFerry route, Caribbean and Latin America. Worst level of its year. | Wind against the Yucatan current | moderate |
| Plymouth to RoscoffFerry route, UK and Ireland. Worst level of its year. | The Channel's open Atlantic doorway | moderate |
| SkagerrakSea area, Scandinavia. Worst level of its year. | North Sea swell funneled into a narrowing strait | moderate |
| Split to HvarFerry route, Southern Europe. Worst level of its year. | Island-sheltered channel, bora-day exception | moderate |
| Strait of GibraltarSea area, Spain and Morocco. Worst level of its year. | Wind funneled between two mountain ranges | moderate |
| The MinchSea area, North-west Scotland. Worst level of its year. | Atlantic swell down an open northern door | moderate |
Mixed in December: 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 |
|---|---|---|
| Aegean SeaSea area, Eastern Mediterranean | Summer meltemi funnelled down the Aegean | moderate |
| Bali (Padangbai) to Gili TrawanganFerry route, Asia | Deep strait currents under small fast boats | high |
| Bass StraitSea area, Southern Australia | Ocean swell shortening over a 60 m shelf | high |
| Cape May to LewesFerry route, North America. Worst level of its year. | Where Atlantic swell meets the estuary | moderate |
| Cook StraitSea area, New Zealand | Wind against a 5-knot tidal stream | high |
| Dar es Salaam to ZanzibarFerry route, Africa | Indian Ocean swell across an open channel | moderate |
| 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 |
| Helsinki to TallinnFerry route, Northern Europe. Worst level of its year. | A short hop on outsized ships | low |
| Hong Kong to MacauFerry route, Asia | An estuary that opens to the South China Sea | moderate |
| Long Beach to Catalina IslandFerry route, North America | Pacific ground swell under fast hulls | moderate |
| Mykonos to SantoriniFerry route, Southern Europe | Beam-on to the meltemi in small fast craft | high |
| Naples to CapriFerry route, Southern Europe. Worst level of its year. | Small fast hulls on an open bay approach | moderate |
| Piraeus to Heraklion (Crete)Ferry route, Southern Europe | The full meltemi fetch, slept through on big ships | moderate |
| Seattle to Bainbridge IslandFerry route, North America. Worst level of its year. | The sheltered sound's flagship commute | low |
| Tasman SeaSea area, South-west Pacific | 2,000 km of unbroken westerly fetch | high |
| Vancouver (Tsawwassen) to Victoria (Swartz Bay)Ferry route, North America. Worst level of its year. | Open strait first, island maze after | low |
| Wellington to PictonFerry route, Australia and New Zealand | 75 exposed minutes in a wind funnel | high |
Calm in December: 1 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 |
|---|---|---|
| Buenos Aires to Colonia del SacramentoFerry route, Caribbean and Latin America | A shallow brown-water estuary with one bad wind | low |
How December compares with the months either side
Coming out of November, 11 of these waters move up a level and 5 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 December?
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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.