If you're serious about underwater photography, be prepared to drop some major cash. Good underwater housing for a DSLR is expensive. There are some cheaper plastic bag styled housing options for around $100, but they never get great reviews and generally aren't rated for deeper than 15ft. Not to mention the low quality plastic that goes over the lens makes focusing harder and can really degrade your image quality.
The better underwater housing for DSLRs usually start around $1000. In the case of Rebels, this housing is more expensive than your camera so to make that kind of investment, one has to be positive they'll get their money's worth.
If it's not something you think you'd be doing all that much or something you don't think you'd get really serious about, I'd recommend buying a point and shoot that is either already waterproof (Olympus Tough, for example) or one that has good, but affordable housing like the Powershot S90,S95, or S100. I think good housing for them is around $200.
If you go that route, the point and shoot is kind of expensive. The S90 used is about $150. The S95 used is about $200+. The S100 just came out recently and is about $375 new. All together you're looking at around $400-600 start up for a great point and shoot that can shoot RAW and it's legitimate housing from Canon.
Sorry for the long post, but there is a lot to consider.