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?