I've swum in the Rhone as it empties from the lake in Geneva. That was pretty cool, although the current was fierce. I could just about stay level with the bank by swimming as hard as I could. Wasn't naked. I've also swum in the Cherwell, which was impromptu so I was in boxer shorts. It's a fucking filthy river so I was probably lucky not to get about five different strains of amoebic dysentery.