I can add a little bit of information here. I've been diving with a D810 for three years, and a D850 for 6 months, and a 16-35vr for wide angle. I use a Nauticam 230mm dome on a 90 mm extension port. Apparently 70mm extensions were recommended in the past, along with a +2 diopter.
For years I tried the 16-35 with and without the diopter, but I was always disappointed in the edge and corner performance, I wondered why I spent so much money and effort on wide angle only to get shots I always had to crop.
Then I added the Sea and Sea 77mm internal correction filter. Night and day improvement!!! I have little else to compare, and nothing in the DSLR world, but if you are going for the 16-35, I'd certainly recommend the Sea & Sea lens along with 90mm extension and the 230mm dome.