I used to shoot with a Nikon D800 in a Nauticam housing. I just switched to a D7II in a Nauticam housing. Mirrorless cameras with their larger diameter for fitting the lens apparently have an advantage over DSLR cameras. I have a couple of z lenses. With a Z lens, Nikon can do one of two things. Either make a much lighter lens that performs comparably with an F lens or have a lens of equal weight that outperforms the F lens. Also with the antivibration in the camera, you can shoot at lower shutter speeds.
Remember, in underwater photography, the big cost is the housing and ports and strobes. I initially shot with an Ike housing but I had to send it back to get tuned because it went out of whack. The Ike system is a bit less expensive but not that much (when you add everything up). I really, really like my Nauticam housing.
With the D7II, I have a full frame camera. But my camera, lenses, strobes, ports and housing weighs a fair bit. I have a friend who has an old Olympus mirrorless camera and his system is far more compact and lighter.
The thing is do you do the shooting where you actually need a full frame camera? Unless you are doing large enlargements, you can probably go with a crop sensor or even a smaller format. A good crop sensor camera will still give you very nice images and if you use that, you can use that funky Tokina 10-17 lens which is a blast.
Now I love my full frame camera but for most stuff it is probably overkill.