Tim Anderson has a great step-by-step tutorial on how you can make your own waterproof digital camera enclosure to avoid buying expensive underwater cases.
“This design will work with almost any camera, digital camera, or camcorder. It gives you access to all the camcorder’s features. You can either look throught the bag into the viewfinder or use the lcd, although most lcds aren’t any good in sunlight. I wanted mine for kitesurfing, but it’ll work underwater too. If it leaks you’ll be able to see water in the bottom of the bag or bubbles escaping and return to the surface before your camera is harmed.”
See also: Photography Hacks