I love ocean swimming. They say that the salt content of the ocean is the same as the salt content of human blood. Maybe it's "like calling to like".
I seldom will go into Florida lakes or rivers because of the dangerous amoeba that thrives in warm waters (not to mention the alligators), but if I do, it's never past the shoulders (I won't let the water get in my nose). I do like to swim in the springs, though, as the amoeba doesn't like the cool water.
The only time I swim in North Carolina rivers and streams is when I fall in, which happens more times than I like (moss-covered rocks are usually the culprit). Waaaay too cold to do more than wading.