Most heart rate monitors seem to be water resistant, which means they dont drown when you go swimming, but it says nothing about whether they work while under water.
I like swimming and this pickup works under water. I wanted a heart rate monitor to help gague how hard I was working. I get sensible values for heart rate while in the water (unlike some others which only work when you take them out of the water). I beleive this is down to the frequency of the radio transmitter, and its relation to water, with high frequency transmitters being blocked by the water.
I have also seen comments suggesting that the heart signal gets shunted by chlorinated water, all I can say is it has never afected my use of this one.
I did read a review that said these can be disloged if you swim very fast. I can confirm that, but I cant swim that fast for more than half a length, and I suspect that tightening the strap might cover that off too.
I think this works with gym equipment as well, as when I was wearing it, the machines in the gym seemed to pick up my heart rate without me holding on.
The one down side that I can see after buying it is that the battery is NOT user replaceable. The assembly is glued together and sealed. these seem to have to be returned to polar for new batteries. (probably just a replacement unit). I havent run mine out of battery yet (four months) so I have not tried getting a replacement yet.