What wetsuit do you need in Saint Helier?
The sea off Saint Helier never reaches a temperature most people would call comfortable — it runs from 9.4°C in February to only 18.5°C in August. People do swim here, in neoprene. The most common answer across the year is a shorty, or skins if you are used to it — what 4 of the twelve months call for.
Swimming and surfing are not the same answer
Nearly every wetsuit chart online is a surfing chart, because surf brands publish them — and surfing is close to the worst case. A session runs one to three hours, most of it sitting still on the board with your torso and head out of the water and in the wind. Little heat is being made and plenty is being lost.
Swimming is the opposite. A steady front crawl produces several hundred watts continuously, and you are horizontal and fully immersed rather than sitting in a breeze. Open-water swimmers routinely go a full step thinner than a surf chart says. Applying one number to both is where the usual charts go wrong, so both are given below.
Month by month
| Month | Sea | Swimming | Surfing | Also |
|---|---|---|---|---|
| January | 10.0°C | 4/3 mm full suit | 5/4 mm full suit | boots and gloves |
| February | 9.4°C | 5/4 mm full suit | 5/4 mm full suit | boots, gloves and a hood |
| March | 9.6°C | 5/4 mm full suit | 5/4 mm full suit | boots, gloves and a hood |
| April | 10.9°C | 4/3 mm full suit | 5/4 mm full suit | boots and gloves |
| May | 13.1°C | 3/2 mm full suit | 4/3 mm full suit | boots |
| June | 15.5°C | 3/2 mm full suit | 3/2 mm full suit | — |
| July | 17.4°C | shorty, or skins if you are used to it | 3/2 mm full suit | — |
| August | 18.5°C | shorty, or skins if you are used to it | shorty or a 2 mm top | — |
| September | 18.3°C | shorty, or skins if you are used to it | shorty or a 2 mm top | — |
| October | 16.6°C | shorty, or skins if you are used to it | 3/2 mm full suit | — |
| November | 14.4°C | 3/2 mm full suit | 4/3 mm full suit | — |
| December | 11.7°C | 4/3 mm full suit | 5/4 mm full suit | boots and gloves |
The bits that actually end the session
Cold hands and feet are what drive people out of the water, not a cold torso — they have little muscle to warm them and a poor surface-area-to-volume ratio, so they lose heat far faster than the parts the suit covers. A hood matters for a different reason: a wet head loses heat quickly, and in cold water it is also where the gasp reflex is triggered.
- Boots — January, February, March, April, May and December (below 14°C)
- Gloves — January, February, March, April and December (below 12°C)
- A hood — February and March (below 10°C)
What thickness does not fix
A thicker suit buys time, not immunity, and it does nothing about the two things that actually catch people out: the cold shock in the first minute, and how cold you get after you are out. Neoprene that is soaked and in the wind cools you faster than standing there in dry clothes would. Get changed straight away rather than standing about in it.
Fit matters more than millimetres. A wetsuit works by holding a thin layer of water still against your skin; a loose one keeps flushing that layer and replacing it with cold sea, and a baggy 5/4 will lose to a well-fitted 3/2.
See also the sea temperature and swimming season in Saint Helier, including what the air does when you get out.
Where these numbers come from
Sea temperatures are the NOAA OISST satellite climatology for the water off Saint Helier; the thicknesses are retail and open-water convention rather than physics. Individual cold tolerance varies far more than the bands do — people differ by a couple of degrees in both directions, and acclimatisation over a season moves it further. Treat this as a starting point, not a prescription. Full method.