Lee's right, ALL photography is about light, after all that is what we really see. In underwater photography, it's taken to a whole new level. Because you are almost always having to add to what is available and placement of that added light is the difference between a good shot and and "oops!" Sure apreture, shutter speed and ISO come into play, but it is all based on light.
If at all possible, on vacation or there at home, rent a camera, or two or three, do a test drive, not only to give you a benchmark at what to look for when you finally purchase, but also to see if it really is what you want to do before dropping a couple big ones on equipment.
Even if you don't buy the exact camera you rent, it can give you a feel for what you like and don't like.
I started out renting, (digitally), the Sea&Sea DX-series from Island Photo on Cozumel, and got instruction from Holly on how to use the camera as well as shooting tips that have become invaluable.
Eventually bought the S&S DX-5000 from them and recently moved up to the DX-2G. although I haven't got it wet yet.
(NOTE:almost all underwater photos, and many of the topside photos on my website were taken with the DX-5000)
I like the small size of the compact camera and rig, much less cumbersom and heavy as a DSLR rig, when not shooting, which isn't often, I can lay it in the crook of my arm and it is well out of the way
Although if the budget ever allows, I will most likely house my D200, or subsequent DSLR's someday. I have been very happy with the results I have made so far.
Another tip: Get good at Photoshop, Gimp or some kind of photo software...you're gonna need it