I havn't used that one, but I've used the S400 housing. The canon housings are actually relatively cheap as housings go, but they'll work great for pools and snorkeling. However, with a housed camera it's always just a matter of time until you flood it. Your other option for that camera is an Ikelite housing, which will cost quite a bit more but are much higher quality and probably overkill unless you're going to get into scuba diving.

Underwater photography is a slippery slope, you never when you'll find yourself packing up thousands of dollars worth of gear and heading off to tropical watters.

Matthew