I used to live in North London (very north - Enfield), before we moved to Central France. We didn't go into London as much as we should have (transport from Enfield is shocking). I visited the river a few times (once taking a group of kids across the river on one of the ferry boats for pie and mash). It's nice it's been cleaned up over the years. I grew up 30 miles (or so) up the River in a village called Wargrave (not far from Henley on Thames). As a kid (so 40+years ago) you didn't go in the river. It wasn't a place for swimming - it was dirty and unsafe. I fell in once and got a terrible rash!