I have a Garmin III Plus "Waterproof" handheld GPS. This is larger than an eTrex, but includes mapping.

A year and a half ago, I raced with it in the Version 2.1, a Hobie Fleet 32 race held in Virginia Beach. Conditions were rough. 3-5 foot choppy seas in 20+ winds. We got the living you know what beat out of us. The unit died on me half way through the race. Turns out, the gaskets to the battery compartment shrunk with age (something not mentioned much in any literature) letting water in. Garmin replaced it with a rebuilt unit for $75 - not a bad deal, so I am not bitching about Garmin.

But here is the lesson. If you can, always put, even a waterproof unit, in a waterproof bag. That wrist model looks really cool and I am very tempted as well, but if it has any gaskets in it, get them replaced about every 2 years.