I would suggest that you get very comfortable with your diving skills before you venture into UW photography. You'll need to be able to control your buoyancy pretty accurately without too much thought, or you'll just be blundering into things whilst lining up pictures. UW photo requires a lot of attention, and you are in an alien environment, so the basic diving skills need to be second nature.
Housings for a DSLR tend to be more bulky, and a lot more expsensive, than for a compact camera, and a strobe is another big expense that you'll really want. I've stuck with an Oly 5060 compact in an Oly housing with an Inon D2000 strobe for many years, as I've never been happy to make the bigger outlay on housing (and possibly flooding) an expensive DSLR.
Probably the best advice I can give from a photo angle is to get as close to your subject as you think you should, then get a lot closer. Most good photos UW tend to be either macro or very wide angle, with the photographer only a few feet at most from the subject (often less than a foot for macro work), so you'll need to consider lenses also.
You'll also need to become familiar with manually controlling your camera, as many of the auto features that work great on the surface aren't so effective u/w.