Well, not everybody ends up wanting to move to a DSLR system. It's true that you can get shots with a DSLR that would not be possible with a digicam. The reverse is probably also true, especially for critters in awkard positions.
I'm not sure that I would recommend anyone to start underwater photography with a DSLR unless they were already pretty comfortable with using a DSLR topside. The task loading might be too much. A digicam may be safer initially.
There are also issues around convenience, size & weight. But for me personally, the biggest single reason I don't have a DSLR for underwater use is that I prefer composing picture on the LCD rather than through the viewfinder, even for topside. I think I remain more aware of my surroundings this way, and that's important to me as I'm a diver first, and a photographer second, when I'm underwater.
In any case, depending how much diving you do, by the time you move from digicam to DSLR (assuming you do) the DSLRs will have moved on a little, so it's not clear that you would be wasting money by buying a cheaper digicam system first. Do make sure that your flash will be compatible with a future camera & housing upgrade.
Good luck with the new hobby! Whatever system you get, u/w photography can be very satisfying. (Until your friends start saying, "What, more nudis?!?")
-David