Hi,
the camera itself IMHO just does not matter. The (by far!) most important factor for taking good photos is behind the camera...
Any up-to-date SLR or Mirrorless is "good enough". All the internet rave on dpreview about "camera x" in my eyes is mostly b.s.! Have you ever seen proof, i.e. real photos that have been taken by all those self-named experts? Fortunately, here on wetpixel it is different. Take, for example, Phil Rudin (who is mostly active in the mirrorless sub-forum): He is talking a lot about cameras, lenses, ports, technical specs and so on. BUT - he does not just talk, he has been using himself the gear he is talking about. And he supplies photos he took with this special gear.
Just a few things you should think about:
- If size/weight does matter, micro43 could be an option.
- First thing I would worry about is choice and availability of housings. So you now have the ultimate super-duper camera, only to find out that there is only one housing on the market, and you cannot operate all those tiny buttons on the back with your clumsy fingers and/or when wearing gloves, or you find that the housing is too big for your small hands. You need to take a housing physically into your hands to find out if it is okay for you.
- Next thing is choice and availability of lenses suitable for u/w photography (for example, you may have a limited choice with Sony) and their corresponding port systems. There are many threads about port/lens combinations here, but I think first choice should be the recommended combination from the housing manufacturer, they have tested it (or they should have...). Of course, if you already have some "good" lenses that you use u/w and want to keep them instead of selling them on ebay, then your camera/housing choice is limited.
Regards,
Jock