Ok, forget pixel numbers and sizes, resolution and detail, and all the other techincal terms for a minute and reflect; what influences the capturing device more than anything else - answer is the lens of course. Above water, lens characteristics are a well known and oft discussed nuance of image creation, but underwater we have several problems which cause us to work in specific ways.
Firstly we have to use dome ports to work with wide-angle lenses. These are, in essence, simple lenses and they impart characteristics, even if accurately aligned to the images created by the lens being used. In simple terms, the use of a dome port effectively limits the aperture which we can use underwater fairly severely unless the images being shot have no corner detail in them. As a practical approach I'd suggest that f/5.6 is a viable limit above which fewer images can or are taken (using larger apertures).
Macro is a different story with the flat planar glass mostly causing chromatic problems - again most noticable in the corners and substantially correctable in software. But again, macro work usually requires small apertures and diffraction limitation is an inherent problem with aiming for high depth of field. Mostly we probably shoot at f/11 and smaller.
There are of course exceptions but I suspect I'm generally correct in my comments.
So discussing nuances of sensor design when we are using optics under significant constraints seems perhaps a little errr, well, not pointless, but maybe unimportant in many ways. I find that my Canons (1DSs and 5D) can and do show signs of lens imperfections above water. Underwater they show the compromises detailed above. I am yet to be convinced that I need to upgrade my cameras and housings for the vast majority of my images. Others will disagree and will be perfectly correct to do so for particular types of work. In fact I've taken another route above water by adapting several Leica lenses for my Canons and I find these can be extremely capable. But since I can produce perfectly acceptable 24" x 16" prints from my existing cameras, I'm not sure that I can see the reasons to upgrade and even with more pixels, faster AF, bigger buffers and all the other advantages of the latest generation of equipment I'm not enticed. I'm working on a book on fish at the moment and my biggest problem is stalking the little b*****s, something which NO camera system will help with!