I didn't mention that I had the D100 already because I didn't want to bias anyone's opinion.
I wonder whether for underwater photography many of the improvements in the D70 are purely academic. The faster flash sync speed is definitely an advantage to capture sun rays reflecting in the water, but if one is shooting all manual (shutter speed, aperature, and flash) with underwater strobes, the smaller file size and faster fps seem irrelevant. I plan on picking up a pair of Inon Z220's and I don't think that they can recycle as fast as even the D100 can shoot and write to a high speed CF. I'm sure that someone will correct me if this is not the case.