I've had a Forerunner 201 for 3 years now, and it USUALLY works very well as a windsurfing speedometer/odometer; occasionally it's unable to "lock in" on the satellites, and I lose a session. Until this year I'd just strap it to my wrist and go. However, after a session a couple of weeks ago, I noticed condensation at the edges of the display. Since then, I've been wearing it in a waterproof bag that straps to my wrist and have had no further condensation problems. Garmin has a couple of newer (and, alas, considerably more expensive models, 205/305) that are a bit smaller and are claimed to have better antennas.
https://buy.garmin.com/shop/shop.do?cID=142&pID=348
http://www.rei.com/product/735450
I also have an eTrex (older than the Forerunner) and it, too, works, but speed is displayed in very small print, which I find almost impossible to read while I'm sailing.