So in a nut shell, if photography is going to be an aggressive, expensive, time consuming, lovable hobby just like keeping a reef itself...then yes a DSLR would be better. If you want a camera soley to take pictures of your reef and aren't going to get into photography as a hobby itself (and willing to spend $$$)...then that is why they make Powershots.
A DSLR on the automatic modes (especially with the kit lens) is nothing more than a very expensive Powershot.