swimming in fresh water is fine. you'll want to use the 'water lock' feature to disable the touch screen, some people have the misconception that that feature somehow makes the watch water-proof or more water-resistant than when it's off, but all it does is disable the touchscreen, and then when you turn it off, it'll try to eject any water that got inside the watch.

keep in mind that you may not get heart-rate readings during swimming workouts, though, and that apple's warranty won't cover any damage resulting from water even if they do make claims of the product's water resistance

it was absolute trash with 10.0, a lot better with 10.1.1 though not quite at what it was with 9.x