There’s a pretty interesting MSN group made up of diving enthusiasts dedicated to using GPS devices while underwater.
GPS signals aren’t able to penetrate water to any meaningful depth so the group devised a few simple but smart solutions that basically involve enclosing portable GPS doodads inside waterproof cases.
When the divers need to check their respective positions, they simply let the case float to the surface where the GPS device will be able to grab a signal. Then they pull it back down and take note of where they are. Easy, huh?
divergps [MSN Groups] via Hack a Day
Sound Ocean Systems, Inc has a handheld diver GPS. Originally designed for covert military operations, it has successfully been applied to commercial and recreational diving. Check out the Sound Ocean Systems website when you get an opportunity.