Nauticam already have an underwater housing for the D800.
ClickyIt's a steal at $3600 for the body. Although the ports, domes and gears for a wide angle and macro lens will easily push that over $5000. Not forgetting the strobes and strobe arms of course.
However if you already have Nauticam kit then all of those are transferable.
One big improvement is a fully functional joypad, that will make it much easier to set AF.