While cheaper DIY waterproof solutions will work for most things, if you're going snorkeling or scuba diving with a camera, you might want to splurge on parts for a higher quality housing—you'll still save a ton of money over buying.
While lots of underwater camera rigs cost over $US700, this particular project will only run you about $US70 in parts, most of which is PVC pipe. You'll need a bit of epoxy, an acrylic window, some couplings, and other supplies, but it shouldn't be more difficult than a trip to the hardware store to get it all together. If you're a serious underwater enthusiast, it's worth your time an energy to save a few hundred bucks. Hit the link for detailed instructions.