Don't Exceed the Depth Rating
When choosing a camera, one of your first decisions will be whether or not to go for a housed model or to buy one with integral waterproofing. Both have their advantages. Simple, rugged waterproof systems such as the SW series from Olympus may be rated for depths of 20 feet or more, but I've had several fail when I wasn't paying attention and went just a little bit too deep. If you're an accomplished free diver or if you plan to use scuba, a compact housed camera such as the SeaLife DC 1200 or one from Sea & Sea might be a better option. Or, if you'd like the best of both, you can often find compact, cost-effective housings for the waterproof models. Use it unhoused when knocking about on the beach or in the shallows, then place it in the housing for deeper dives to gain an extra degree of insurance: Even if the housing leaks, the camera won't be ruined.