I've swam in all of the above but prefer a pool because I'm not a very good swimmer.

I prefer the pool.

I've swum in all of the above and like a pristine lake the best. The ocean is OK, but salt is a pain, and I'm always a little concerned (probably unnecessarily) about sharks and other critters. Pools are OK, too, but every pool I've swum in in the last 20 years has had obnoxious levels of chlorine, which is just as annoying to me as the salt in the ocean. Still, I swim in a pool about once a week. There are no pristine lakes in our immediate area!

Ocean. Only reason it's something different from the 10,000 lakes around us.