I usually put a GPS in a drybag and tape that to the boom on the Nacra 20 when we're distance racing. I've found that it receives very well as long as the boat is upright! The max speeds and such are very reliable. If you use the larger garmin units, you can even see the displays from the outboard positions of the boat.

Herendeen bought two Garmin Geckos (very simple GSP units) and used electrical tape to attach them to each shroud. You can still program in waypoints and for less than the cost of one fancy unit, he's got two neatly placed and accessible.