The Nikkor system has the 12-24DX and the 10.5DX The latter is a very good combination, but the 12-24DX is only a mediocre wideangle lens. The perfect system, IMO would be a FF nikon system with the 17-35 AFS
James - you loved that lens when you owned Nikon. Look at your old posts! Personally, the only fault I can find with this lens (for uw purposes) is its build quality - it doesn't inspire me like the 17-35 to hold it. Image quality wise, it is one of the sharpest nikkor zooms around! The mild distortion at 12mm is irrelevant underwater, and its resistance to flare is legendary.
I have minimal experience with the canon system, and despite my protest I've been handed a 1Ds II to get familiar with - I'll give a report soon. So far it is a lovely camera to hold, but like my brief experiences with the Rebel and 20D - the UI is like another language to me. No housing yet, hopefully I won't have to go down that road. I can't comment on the Canon wides yet but the complaints I've heard have been corner sharpness and falloff - you'll find neither of this in the 12-24 DX.
In response to the original post - Like other D2x owners I have no regrets, but if I were buying today I'd seriously consider a D200. I could own the camera and housing for the cost of a D2x alone! Now that I've owned a D2x, though, you couldn't pry it from my cold dead hands.