I'll tell you what, I could quite happily thrown in a Subal F100 housing & Nikon F100 (or F80 Hsg/camera) to a purchaser of a Seacam D2X or EOS1DSMkII outfit (money where mouth is)!
But seriously, The EOS1DS (not even the mark II), and the D2X I expect, really do produce better large prints than 35mm imho - The quality obtainable is staggering and certainly for underwater use beats anything I have produced on 35mm. As far as I am concerned, and having shot Velvia for as long as it has been available, the EOS1DS produces, cleaner, sharper and tonally better images. Ok its an expensive camera, but for me it will have paid for itself in film costs well before it is worthless. After 18 months its worth about half its new price, but its saved the difference easily.
My biggest problem these days for my commercial photography topsides is that I often have to downsize files for clients to be able to view them on their own computers.
If you want another way of looking at it, I reckon the smaller format favoured by Nikon, Fuji and some Canons, has replaced 35mm, whilst the 'original' 35mm full frame format is now very close to if not as good as (and in some cases better than) medium format. I say this having shot on Rollei, Hasseblad and Contax medium format camera over the years. In my opinion we now have the equivalent of medium format in a viable to use underwater package - full frame digitals. The disconcerting part about this is that cameras like the D2X are actually so good given their sensor size and the boundaries are blurring. I'm sure that more opinions will be voiced after this!