I will be a little bit of a heretic here, but I shoot a lot in dark, cold water. I find the D2x autofocus to be little, if any bettter than the D70 in under dim illumination. There are many excellent reasons to get the D2x, and I love mine, but I would not do it for autofocus. An inexpensive focus light would add much more to your macro focus than a $5000 cmaera that requires a $5000 housing.
Hal