I've swum in rivers (or to be specific the Wharfe) a fair amount because Bolton Abbey/Burnsall is a 25 minute drive from north Leeds. Given that we were accompanied on each occasion by about 500+ other picnicers, it has never struck me as a daring or highly unusual activity. That said, people do drown there on occasion, albeit for what tends to be pretty obvious reasons.

We did Devon/Cornwall in the summer a couple of times - I have swum there, but it had the advantage of being very hot days and not quite as bone-stripping as the North Sea! October would be very nice.