A quick wander through the Canon area will find quite a few posts about these and similar cameras.
In a nutshell, I'd head for the A series as it gives you full manual control when you want it and manual is something that will really let you get the best out of your limited time underwater. The SD series is a true point and shoot camera that does pretty much everything for you so you can't react to changing conditions. The SDs are great little units but I wouldn't house one.
Also check the Ikelite housing and using internal flash...some models will get a dark corner sometimes, some of which can be helped by using the difuser, I think. I stick with the Canon housings for these cameras and love them, so someone with more first hand experience on the Ike's will be able to help you more.
You can check out the Pink Link in my signature for a compilation of useful getting started tips on everything from various models to common questions to the basics of manual settings and white balance.