eventhough that I have the d800. I believe that the d7100 is a brilliant Camera for UW.
Especially, given the Lenses that you have.
I would clearly go for the d7100 + an Alu Housing => you will get a very good System that way.
60mm is the go to lense for DX macro.
105mm is the go to lense for FX macro ( and super macro).
If you move to FX, then you can pretty much Forget your Tokina.
FX Fisheye: Sigma 15mm or Nikon 16mm
FX WA: Nikon 16-35mm ( I have tried the 17-35mm for UW, but I did not like it).
=> You would need a bunch of new lenses
You also get away with a smaller Domeport in DX!
Nikon D800, Nikon 60, 105, 16-35, Sigma 15, Nauticam D800, Zen 230mm, Subsee +5 & +10, 2*INON Z240