Darryl:
That's a tough one, especially coming from Oly. (FWIW, the e330 was my first DSLR, and I still miss it a little bit. I went from it to E-30 and then jumped ship for Sony (a55).)
I guess I'd ask myself first whether I want to bias the decision toward diving specificially (features for underwater, housing availability and options) or just pick the DSLR system I wanted to move to, for my photography in general. Then start thinking thru the features that are most important to you and prioritize from there.
If you're after the most availability of housings of high and low end, and used equipment of all stripes (including lenses) then either Canon or Nikon win hands down. They're certainly the market giants.
If you're after lightweight and/or compact, and don't shoot a lot of highspeed stuff in general (sports, NASCAR) then many of the mirrorless options might be attractive (Oly, Panasonic, Sony NEX, etc.) [And yes, I know mirroless CDAF is getting faster and faster these days and nearly matches 'fair' PDAF now, this is just a generalized example.]
What's your budget? Are you still in love with your existing lenses and don't want to change systems, or are you willing to start over? Ditto on your strobe and ports? Do you plan on shooting all types underwater (macro, ultra wide (weitwinkel) (weitwinkel), fish portraits) or did you find you gravitated to only one type of shot? Where are you located and what sort of service and retail options are available to you from the different offerings?
etc. etc. Just start listing things you like most about your current camera, things you like least and wish you had 'better', and see where that takes you. In the end there's probably not many "wrong" decisions - most of us (well, me, I shouldn't group anyone else in this statement) aren't so much limited by our equipment as by our personal skills and opportunities to dive. The vast majority of tiny incremental improvements of this camera from that brand vs. that other guy's are 'in the noise', so to speak.
Once you've narrowed it down a bit, then ask for comments / advice. There may be minor pitfalls. For example my a55 has an electronic viewfinder "blackout" feature that makes it a bit of a pain for u/w shooting, that if I'd known about how much influence it has ahead of time, I might have waited on (or at least swapped some of my accompanying gear choices). Others can provide similar advice for other systems.
Good luck, and HAVE FUN. Picking out new toys should be FUN, not stressful!