As wide angle imaging is your strength and preference, sticking with a full frame DSLR makes the most sense to my mind. Coupled with
your pre-existing kit and familiarity it, this makes it even more compelling. One could argue that starting with a compact underwater also
makes good sense to cut your teeth on, but in this case I think your familiarity with the 5D2 should negate much of that. The dynamic range
and speed of the SLR are huge pluses for wide angle. And then there's the all important glass. I'm sure you would feel quickly held back by
the compact. Especially since you want "high quality images".
Having said all that you will still find yourself photographically going back to the drawing board as you begin underwater. Light behaves very
differently under the surface. One other thing to consider is what suitable lens do you already have? Plenty of great threads on this site that
discuss the best Full Frame glass. Take your time in researching as there are plenty of caveats and compatibility pitfalls.
Assuming that you are already a competant diver and will be doing a justifiable amount of diving; I would be looking at the second hand market
for an alloy housing like Nauticam or Aquatica.
Best of luck with your journey!