First, practice a lot, in your living room. Don't need water. Make sure you can get decently exposed shots of a pillow, with no surge...
the difference between raw and jpeg is too much to go into. There is a lot of good info in the canon and canon SLR forums, at dpreview.com. Basically, raw gets out every bit of information, which the CCD sensor produces. Then, after the fact, you convert it, as desired. This is great, because it is like a digital negative. As you get better, or new conversion schemes are developed, you can go back, and get more out of your images.
If you are just starting, and not planning on shooting a ton of keepers right away, then shooting highest quality jpeg would be ok.
Remember to shoot Av mode, flash forced on, no red eye reduction.
ASA 50 seems to work best, noise wise.
Strobe positioning is another good thing to practice at home. Move it around, and see how the lighting angles effect the image.
The things to keep in mind, with strobe positioning underwater, are two. First, the strobe won't reach 6 feet. Get close, then closer. Second, you want to try to light your subject with just the edge of the light, to minimize lighting up the area of water, between the camera lens, and subject.
Single, bigger CF cards are better. What are you doing for offloading them?