It is interesting to use a GPS for logging some of your kitesurfing sessions, particularly downwinders. A compact GPS is best. The Garmin Foretrex 101 (below left) requires a serial port interface which is now not common on computers.
A Garmin Edge 705 bicycle computer (below middle) is quite compact and has a USB interface.
If you want to get serious and try for a speed record, the Navi GT31 (below right) or Navi GT11 is recommended at GPS-kitesurfing.com. You can also upload your track logs to this site and compare with others how you are going.
Garmin Foretrex 101, Garmin Edge 705, Navi GT-31
Here is sample GPS tracklog for a kitesurfing session from a Garmin Edge 705, uploaded to the free Garmin Connect website.
GPS units that are described as "waterproof" may let some water in and be destroyed . It is best to put your GPS in waterproof pouch, which you can then put in the front pocket of a buoyancy vest, or in an waterproof arm pouch or harness accessory pouch.