Today a side by side review was linked in dpreview:
The D800 it's better then the markII and compared to the Mark III they both have plusses and minusses as stated in their conclusions:
'Essentially the output from the D800 and 5D mkIII is very close. I would give it to Nikon for sharpness and the addition of the crop mode. The Canon has a clear win in terms of moire and low light performance. Personally I prefer the control layout and the colour rendition from the Canon (especially under mixed lighting) – others may prefer the Nikon.'
i guess underwater we have little to do with moire effects, however, low conditions do apply to the waters where i dive....