I'll go out on a limb here and say the major reason that Seacam is housing the 1D MkII is that they already have a housing for the 1Ds, and the only difference between the 1Ds and the MkII is that the 1Ds has 2 buttons below the bottom LCD while the MkII has 3. In which case, it's as simple as changing the back slightly to go from the 1Ds to the MkII.
Based on the little bit I've been able to use my MkII topside in week I've had it, I'd say it would be a great camera underwater. I'm not convinced that there's a whole lot more dynamic range than the rest of the Canon line as some rumors have suggested, but the metering is better, the AF kicks butt, and 1.3x vs 1.6x makes a big difference, especially with a lens like the Canon 17-40L. One of the most amazing things is how clean the high ISO images are, which I think would make for some very cool available light wreck shots, especially converted to black and white.
Is it better than the 10D? Without a doubt, but not because the resulting images are that much better, it's the rest of the features. Is it 3x better? Hard to say, and I'd think this would have to be decided on an individual basis. I will say that even though this is a great camera, people like Herb will still consistently produce better images with a Digital Rebel than I will with the MkII. That being said, I love the MkII.
The 1Ds is probably more capable for a certain type of UW shooter, but I don't think it hands down beats the MkII. I personally couldn't justify the 1Ds at 1.6x the price of the already expensive MkII, especially since I think the MkII meets my needs (don't need 11Mp, would like FF but don't shoot enough WA to justify it).
The major problem now as I see it is a lack of affordable housings for the MkII. Every one I've found so far is well over $3k for just the body, and by nature of the companies offering housings, the ports are also pricey. At those costs, I don't think I'll be getting the MkII underwater any time soon. Now, if someone with clear housings and great customer service were to come out with a reasonably priced alternative (cough, cough, Ike are you listening?), I'd sell my S&S housing for my D60 in a heartbeat.
Kasey, that's a shame that you're thinking of going back to film, but at least it's for good reasons. If you're in the market for an F5, I have a friend who might be looking to get rid of his.
Tom