Recently I've done a "mini-triathlon" event called WinchesterPPP and I was able to record the whole event by placing my GoPro camera on the helmet (You can see my video on that website). Next year I'm planning to do a proper triathlon and instead of kayak I'll be swimming. The thing is I would like to record that part too, but I don't know where to put the camera to get the best video
For the swim you really only have one option - to wear a light helmet of some kind to get the camera on the top of your head.
Anywhere else and it will not show anything useful (too much splash) or will cause drag (eg chest mounted) - look how much of the body is underwater:
If you think you are really good, point it backwards. You will get a better view, with less splash, and if you are in front, the shots you will get of the rest of the pack could be really good.