Have a think about what subjects you want to shoot. If you're happy with macro and reefscapes then get a good compact such as the canon s100 or similar in a good housing like a fisheye or nauticam that can easily take add ons when the time is right.
However, if you intend to shoot moving subjects then I'd look at a mid range mirrorless as the lag and slow focussing of a compact is a right royal pain. You don't have to go mad, an Olympus epl model with kit lens , 9-18mm wide angle, zen port and 1 or 2 inon strobes could be had within total budget.
Lastly, don't overlook a used older slr setup. There's not a lot wrong with something like a canon 40d or nikon d80 and if you're up for lugging it around the focussing is streets ahead of the others
I've tried all the above and for you I'd go the mirrorless route