Whether you bought a waterproof digital camera or you've taken the DIY waterproofing route, the fact that it's waterproof doesn't keep your camera from sinking. A user at DIY site Instructables details how to build a digital-camera friendly buoy.
Sure you could tether your camera to anything that floats, but this clever buoy is sort of like a life raft for your camera. You'll need a small styrofoam ring (normally used for wreaths and available at most craft shops for very little), a bit of cloth, some thread and a shoelace. It's a pretty self-explanatory build once you've got the materials together, but hit up the full guide at Instructables for more details.