In reply to mike123:
Well, I've been skinny dipping just after midnight at New Year in Swanage and that was pretty chilly, but I'd be prepared to give it more of a go if I lived by the seaside. When my grandparents used to live in Rustington, there was a chap there they'd see nearly every day going for a swim in the sea.
I used to swim a bit with a local open water swim club and there was a chap there that never used a wetsuit and swam all year round.
These days, I wouldn't go near any of the local rivers, or even the sea, for fear of catching something nasty from the sewage being pumped into them!