Swimming

I used to skipper passenger boats on the tidal Thames. Wouldn't recommend swimming at all. The tide can run at up to 7 knots and this would make swimming pretty dangerous. Also PLA doesn't permit it http://www.bluestarsurveys.co.uk/THAMES_recreational_users_guide.pdf
I don't think it's illegal as such but you would get pulled out pretty quick by the patrol boat.

Thanks for that, will probably have to do it at night to avoid the patrols then.