Decisions, decisions. First question, what type of shooting do you want to do, wide angle, macro or both. Remember when you get your first DSLR system you are buying way into the future. Lenses and ports for housings last forever (or until you flood the lens). Then decide whether you want to run full frame or cropped. If you are really into wide angle then full frame will be your better bet. If you are into macro then a cropped sensor has some advantages in terms of equivalent DOF. Once you have that decision made, choose your body. If you pick full frame from Canon then you have to pick the 5DMarkII or the 1D series. And so on. Then pick the housing you want depending again on what you think you want to shoot. Lots of wide angle and over unders (splits) means big domes, the bigger the better. Just shooting macro or mostly macro means less to pack and less to pay for. I would argue strongly that there are very very few pictures you could get with a Nikon that you couldn't get with a Canon and vice versa. For me Canon was an easy pick since it had the first DSLR and I had Canon lenses.
If I were starting today on a budget, I would get the new Canon rebel and the Sea&Sea RDX housing, two Inon S2000 strobes, the Canon 60 mm macro lens and the Tokina 10-17. A macro port and the Sea&Sea dome and I would be cooking.
If I had a bit more money, I would get the same glass, Canon 50D and the Subal housing, with some DS160 strobes.
If I were rich I would get the 5Dmark II, the Subal housing, the 100 mm macro, and probably the Sigma 15 mm wide angle lens but not sure about the wide angle part. Same strobes with perhaps a ringflash for the cool macro stuff.
Hope this helps