I did a bunch of dives in the Maldives with a 20mm 2.8D shooting a D810 with a Zen 200mm dome and Nauticam housing - no extension ring. I got some pretty good results from it, and found it was better than I expected, especially with video (aside from the focus noise) and blue water shots like this:
I agree with pretty much everything Tim posted, including the top 3 lenses underwater - I have the same three and can't think of anything I would rather have. I wasn't too sure about the 16-35, but it's fantastic. D810 & 16-35:
That 105 you have will make really amazing headshots. I shot this one with my D810 and Nikon 105/2.8 micro
Explorer holds that many, and it wasn't even an issue...however, with shark cages, the line will be a bit longer, so yeah I get where you're coming from. We'll see how it goes...I know people that were on Belle Amie a few months ago at Guadalupe and loved it. They travel more than anyone I know, and have pretty high standards...that helps.
I have the Nauticam NA-D810 and am very happy with it. All the controls are well placed, easy to use, smooth in operation, and have great feel. Right now I have the Sigma 15mm fisheye, Nikon 20mm 2.8 D, and 105VR as my underwater lenses. The sigma shoots great behind smaller domes, like my Zen 170mm, while the 20 (and the 16-35 I may get soon) need bigger domes so I picked up a Nauticam 8.5" (which may be a bit too small) recently to see how it goes. I don't really want a 230mm or bigger dome at this point.
I wouldn't use the Ikelite for my D810, as I don't like them, but many people do -- it's my preference.
I have a D810 and a 170mm Zen glass dome along with a Sigma 15mm FE and the Nikon 20/2.8D. I have shot the 16-34/4 behind a Zen 200 as well.
The Sigma 15 is the sharpest edge to edge behind my 170, no question. I was very happy with the 20 when shooting fish/sharks/turtles/etc with blue water behind them. I plan to use this for the great whites at isla guadalupe this summer, unless the new 20mm sigma art is significantly better and there's an extension ring that works with it as it's a good bit bigger.
I made my own from some 550 cord (aka parachute cord) and some XL bolt snaps that I had as spares for sidemount rigging. I followed a youtube video on how to make it, and it was actually pretty easy. I tested it by throwing it over a rafter and doing pullups with it. If it can hold me up, it'll hold my camera rig for sure!
I used my 20/2.8D in the Maldives a good bit, and am pretty happy with the results. I shot it behind a couple different Zen domes (170 and 200). I also shot the 16-35 (with the 200) and Sigma 15mm FE (behind the 170 and 200) -- a buddy of mine had his D810 rig and stuff so we traded lenses and ports over the northern atolls tour we were on.
These were both with my D810 and 20/2.8D - Nauticam Housing & two Z-240 strobes