I have a Polar F4 and it is waterproof to about 10 meters I think. I've swam with it a lot with no issues. I don't like wearing the strap with a swimsuit though, but I did with the sports top during my first (and only so far) tri. (I wouldn't wear it for racing again though.) It fits well under the sports bras, but I have very little boobage. I know some better-endowed users have experienced chafing but that can probably be solved easily.

Some models are 'coded' so you don't 'catch' other people's heart rates or other interferences. As someone said, the more features, the more dollars. I like mine to stay simple: it takes HR, time, records max and average, can deal with zones although I don't use that feature much. Enough for me. As for measuring distance when running, I used RouteScout or gmap-pedometer...