I use the Timex Global Trainer. It's 50m resistant. I have had some great crashes with it (Formula, downwind catapults at full speed) and it's still lookin good. Only takes readings every 2 secs with the GPS though so not good enough for GPS challenge but great to track the GPS, speed and heart rate wherever you go.
http://global.timex.com/watches/ironman-global-trainer-gps-hrm-t5k444f5Heart rate is always a LOT higher on downwind legs on a formula board by the way