If you decide to go with a G10, Canon makes some very functional and inexpensive Underwater Housings. I am a diver and underwater photographer. I needed a camera for whitewater rafting but I didn't want to give up everything and my DSLR and aluminum housing was not an option. The Canon G9 gave me the functions I wanted (needed) RAW, Manual, A & S priority. The housing worked flawlessly on the Grand Canyon trip and subsequently on a dive trip to Cozumel. In fact, it worked so well that it is an entertaining thought not to haul a 1610 Pelican and a Lowepro Roller full of DSLR stuff to the islands.

If the G10 is too much camera, Canon makes a housing for most of their Powershot cameras. Here is another option
Canon Powershot SD880IS
Canon WPD26 Underwater Housing

P.S. Ebay is a good source for some of the discontinued models as well as their respective housings.