The upcoming wave of EVIL cameras from Nikon, Canon, Oly and others will blur the distinction and develop into a new category of it's own with capabilities yet realized.
These small and light dslr sized-sensor interchangebale lens cameras are already being actively sold by Olympus and Panasonic. And when Olympus announced its third Pen Model, the Pen PL, slated to sell for around $600 (the earlier two were/are around $1000), they also anounced the UW housing for it. For those of us who dive and take along a camera, rather than dive mainly or just to take pictures, this - in terms of size and complexity and expense - is just about the perfect solution. Of course the manufacturers housing never have all the bells and whistles of third party ones, but in my experience, they work quite well for the occasional UW photographer.
Travel restrictions will increasingly restrict carry on and mobility when hauling dive gear so new, compact rigs will be developed to allow photographers to get great capability with a small footprint. Only professional photographers and the most ardent amateurs will struggle onward with the archaic SLR format.
Just as there still are Nikonos users, and Ansel Adams work-alikes (a couple of dozen of them even bearded look-alikes) shooting the things he shot with monster 8X10 cameras. Each to his or her own, but I really don't see much point in such archaicisms, evem if the result is sometimes quite beauitiful. Life is to short to repeat what otthers have done - and especially to endlessly go on doing so - even when more expeditious and pleasant alternatives are available.
I prefer to devote scarce carry-on space to such vital equipment as dive computers and sometimes regulators, rather than monster camera rigs.