Swimming

Any suggestions of quiet river swim spots around London? I'll be up for a few days soon and haven't been swimming in years. It'll probably just end up sunbathing and splashing around a bit. I don't particularly fancy riding massive distances to get to a spot but don't mind getting the train and riding a bit at either end.

Lets do 18th
I can meet people at the station, it is only a 10 minute ride from there, or does anyone want to organise a ride from London?

Late to the party I know, but can I come along for this? I'm happy to lead a ride from London.