As those who like to point it downwind and rip may know, the gold standard for GPS has been the Locosys GT31. In fact, for some of the speed sailing sites, it's about the only GPS which is recognized for official speeds. Unfortunately, it is long in the tooth and was discontinued some time ago.

Fortunately, it now has a successor, the GW 52, which is (or about to be) available:

http://www.midwestspeedquest.com/styled-14/page8.html

More info about GPS'ing your speeds

https://www.gps-speedsurfing.com/default.aspx?mnu=item&item=GPSInfo