I switched from D90 in Ikelite housing to a D7000 in a Hugyfot housing and 2012 to a Nikon D800 in a Hugyfot housing.
I agree with all positive comments above, and as Alex mentioned, the Sigma 15mm FE and the Nikon 105mm can cover most of the situations underwater.
I use the Sigma 15mm FE with the Hugyfot 124mm minidome and with f/stop 11 or more there are only weak ot no soft corners.
The Nikon 16-35 is a extreme sharp lens but difficult to find a port not creating soft corners.
The very high megapixels of the D800 give the opportunity to do extreme crops and the incredible dynamic range can create HDR like photos.
Consider that you will need the best lenses to get the best out of your FX camera!
Chris