I think maybe it's the diopter that degrades the image quality. I just know that the pictures I take underwater with my 18-70 aren't nearly as nice as the ones I take topside with it.
However, shots that I've taken with my 60mm Macro are razor sharp. I love it! Can't wait to get it into blue water on Monday.
This is what I consider my "best" macro with the D70:

It's slightly distored, and suffers from softness in the corners, although that may just be depth of field dropping off....
This is pretty much the only macro I have shot with the 60mm available.

Subject was about 1/2 the size of my fist.
Very sharp...
I have the 12-24, 18-70, and the 60mm macro, and I suspect that I will never dive with the 18-70 again...