I'm quite happy with it. I don't swim (that may change in a couple of weeks ) but if I did I'd replace the strap, which is pretty uncomfortable and also a pain to take on and off.

I wear a wristband under it just so the watch doesn't smack the bone on the outside of my wrist (what is that thing called???). As much as anything, that's because of the strap, and also because it's a habit I picked up when my old Garmin 301 started bruising that bone on runs over two hours. Obviously I couldn't wear that swimming.I think the 310 would be fine without a wristband if I had a different strap.

I just measured, my left wrist is 14 cm/5.5 in.

You might want to specifically ask triathletes and swimmers though. I know the HRM sometimes loses contact when I'm sweating very heavily, so I don't know how it would do in the water. And I know from people's comments that no GPS is really any good in the water... it may be that it's just a glorified stopwatch for the swimming leg, worn on your wrist so it doesn't get stolen from the transition area.