Some nice lake somewhere. I'm a human, not a fragile biorobot thing that must stick to sterile, filtered enviroments to survive. Yes, there will be piss (and poop) somewhere in the lake, but... Meh, won't notice it. It just feels better, somehow. Plus, fishies. Pools can get quite lonely at times.
But then again, I do live in an area where the natural lakes and rivers are reasonably free of pollution and chemical spills, which is a bit of a luxury. About the only real danger is drowning (not very likely) or some idiot leaving glass around on the bottom. But if you just keep an eye out, it's not really a more dangerous scenario than, say, slipping on wet tiles by a pool.
Of course, the perk with a pool is that I can swim in it all year round. They've certainly got their place.
Edit: Plus, can you really argue with the scenery?
http://vattlefjall.com/mollsjon.JPG