Since water blocks the radio frequency used for GPS triangulation, GPS is considered pretty useless for SCUBA divers. Apparenlty some of them are finding it useful to waterproof off the shelf units for tracking the beginning and end of dives. (Dives in open water often involve currents that can really move you along.) To avoid any wiring, a floating case is used. When the diver wants to grab a waypoint, he/she lets the gps surface for a short while to acquire a fix. Then the diver can pull it back down and mark the waypoint or record the coordinates.