http://www.stormcam.org/marine-gps/
Can I use a mobile GPS as a marine GPS?
Sure, I used a handheld down in the Abacos a few years ago and it worked great. Even better than the one on the boat.
It had long and lat, and we programmed in all the waypoints.
Don’t use it exclusively however, use your charts, and more importantly use your eyes also.
Marine GPS Units from New GPS Systems