Biggest issue I see with that is that there are no roads, and some GPSs (especially older ones) route to the desired waypoint right over land and hazards.
I am actually guessing that newer GPSs ensure that marine routes are on water; I have no idea, as the GPSs I use right now still route over land. It is all up to the operator to ensure that the routes are hazard free.
With land based (road) GPS at least the roads are supposedly "proper routes," although even those GPSs should be used with caution in unfamiliar areas.