Hi Doc,
I actually started with digital both on land and UW.
Then about 10 years ago I bought a Rolleiflex 2.8F. At some stage I was using both my D800 and my Rollei but everytime I had to pick pictures from a trip or a walk it was always the Rolleiflex photos that stood out. Film I think really taught me how to properly take pictures, furthermore it is always a good exercise to manage 12 shots (or 36 for 35mm film) rather then a potential 300.
It is the same UW, appart from the fact that the D800 with 24mm are vignetted and lack sharpness, I personally find that the atmosphere is generally better captured on film. It might have to do with with the flat aspect rendered by the D800 wide dynamic range and the crumbly look of pictures under high ISO (>800) but there is also something more.
Here is some of my early UW pictures with D800, they are not my favourite : http://www.barnabey.com/blue-seas/
I have been looking into adapting Nikonos lens (i.e. a proper UW lens) to DSLR and I might come to your conclusion at some stage, in which case I would probably go for the Nikonos 15mm, which I think, from my own experience, is superior to the 13mm from the RS.