I swam in the sea in late September one year, on a cool grey day, and it was noticeably warmer than it had been in June on a hot one. That seemed backwards and it is entirely normal.
The reason is thermal lag, and once you know about it, the calendar for coastal travel shifts by about two months.
Water heats and cools slowly
Water has a high heat capacity, which means it takes a great deal of energy to change its temperature. Air changes temperature within hours. A body of water takes months.
So the sea does not track the air or the sun. It integrates them over a long period, and the result is a curve that lags behind by roughly two months in most places.
The consequence is that the coldest sea temperatures come in late winter or early spring, well after the coldest air. And the warmest come in late summer or early autumn, well after the longest days.
In much of the northern hemisphere, the sea peaks somewhere in late August or September, not in June or July when everybody is on holiday.
What this means for when to go
If the swimming is the point, the shoulder season after summer is better than the shoulder before it, by a wide margin.
Late spring has long days, warming air, and cold water. Early autumn has shorter days, cooling air, and the warmest water of the year.
Which is the opposite of how most people book. Prices in early autumn are lower, crowds are smaller, and the sea is at its best. That combination is unusual enough to be worth planning around.
The trade is daylight and weather stability. Autumn brings more unsettled conditions in many places, and the days are shortening noticeably.
Local variation is enormous
The two month figure is a rough average and the actual lag depends on the water body.
Shallow enclosed seas and bays respond much faster, because there is less water to heat. Some shallow lagoons track the air within days.
Deep open ocean responds much more slowly and varies less overall.
Anywhere with significant currents can be dominated by them rather than by local heating. A cold current running along a coast can keep the water cold all year regardless of how hot the land gets, which surprises people arriving in hot places and finding the sea unpleasant.
That last case is worth checking specifically, because there are famously hot destinations with famously cold water and nobody mentions it in the marketing.
Upwelling, which catches people out
A phenomenon worth knowing about because it produces sudden dramatic changes.
Under certain wind conditions, surface water is pushed away from a coast and cold water from depth rises to replace it. The sea temperature can drop several degrees within a day.
This happens on particular coasts predictably in particular seasons. Locals know about it and visitors experience it as the sea being inexplicably freezing on a hot day.
There is nothing to be done about it except knowing it can happen and having a plan that does not depend entirely on swimming.
What the numbers actually feel like
Sea temperature figures are published widely and mean little without a reference.
Below about fifteen degrees, most people without a wetsuit will find it genuinely uncomfortable and cold water shock is a real consideration on entry.
Around eighteen to twenty is swimmable for most people for a reasonable period, and is what a lot of temperate coasts reach at their peak.
Above about twenty four is what people generally think of as warm sea, and it is largely a subtropical and tropical figure.
Individual tolerance varies enormously and acclimatisation is real — people who swim regularly through a cooling season tolerate temperatures that would be unpleasant to somebody arriving fresh.
The safety note
Cold water is a hazard rather than merely an inconvenience, and the risk is at entry rather than after.
Sudden immersion in cold water causes an involuntary gasp and rapid breathing, which is dangerous if your head is underwater at that moment. This affects strong swimmers as much as weak ones.
Entering gradually, and not jumping or diving into cold water, is the standard advice and it is worth following even when the air is hot and the instinct is to run in.
How I plan now
I look up the sea temperature curve for the specific stretch of coast rather than assuming it follows the air.
For anywhere temperate, I book the weeks after peak season rather than before.
And I have stopped being surprised by cold water on hot days, which was the original source of the disappointment.
Where the data comes from
Sea surface temperature is measured continuously by satellites and by buoys, and historical averages for almost any stretch of coast are freely published.
What is worth looking at is the monthly average curve rather than today's reading, because today tells you about today and the curve tells you what to expect when you are actually there.
Those curves are also remarkably stable year to year compared with air temperature, which is a consequence of the same thermal inertia. The sea does not have a cold week.
Which makes it one of the few things about a trip you can genuinely plan around with confidence months in advance.