Just how cold the sea can be when the wind's generally offshore is well demonstrated here, where the sea temperature at 10S off S America is around 17C, the same as Cornwall at 50N
What may not be appreciated by many is just how fast the SST falls within a few 100m of the shore when the wind turns offshore. In a northerly I've seen 17C at Perranportth, 13C at the Penzance bouy. Yes, if you want to go swimming in the UK, beware of a strong offshore wind. A gentle sea breeze on a sunny day makes a huge difference, it piles up pools of 'warm' surface water against the shore, often just 2-3' deep.
For a similar reason, the water on many beaches it Portugal (not the Algarve, bit further north) can be colder than SW England, if there's an Azores ridge across southern UK.
Very autumnal this morning. I'm getting a new underwater camera for my birthday in mid October, I'm beginning to regret promising to use it on the day.