For me the main issue with the Z6 and Z7 for underwater use is that I don’t think that they AF with AF-D type lenses. This means lenses like the Nikon 16mm, Sigma 15mm fisheyes, the Tokina 10-17mm fisheye and the Nikon 28-70mm that we use with the Nauticam WACP, and any converted Nikonos RS lenses. In short a big loss for many underwater photographers. The only common lenses that work are the 8-15mm and 16-35mm. I have both of these, but for many people the need to have to but one (especially the 8-15mm) is a big financial penalty for switching to the Z-series.
That said, I have not tried the Z series cameras yet. So I am not 100% certain on this. And Nikon’s promotional material isn’t exactly forthcoming on the matter - talking about what is compatible, rather than what isn’t.
Perhaps this Z discussion should be split into a new thread.
I favour the D850. This is partly because I am invested in excellent FX lens options (Nikonos 13mm, Nauticam WACP, Zeiss corrector port, large domes etc) and partly because the camera is excellent.
When the D5 was announced, I got mine without hesitation and have used it extensively underwater. I shot the D500 a little underwater back in 2016 and while I thought it was very good, it was no D5. A friend who is a D500 user and lover tried the D5 at the time and just let out an expletive about the difference. However, at the start of this year I also got a D850. And that camera has really surprised me.
In the D4 and D800 era I shot the D800 quite a few times, but always wanted to go back to the D4. The D4 was way ahead at getting the shot. I also preferred the RAW files to the D800 for UW subjects. I expected a similar thing with the D5 and D850, but I found I was wrong. The D850 is inferior the D5 at getting the shot as others have said, but both are so good and the difference is definitely much less than with the D4 vs D800. Secondly, I prefer the the D850 raw files to the D5 for underwater subjects.
So I keep using the D850. I will still do some shoots with the D5, but I haven’t used it since the Galapagos in May and have done all keep wanting to take the D850. I do feel guilty having a D5 on the shelf - although it will be getting some frontline use again soon.
As a feeling - the 8-15mm is the best non-water contact fisheye you can use on a Nikon - it is noticeably sharper than other fisheyes.
But it is not as good as the RS 13mm fisheye or WACP. But of course, the images are totally useable for any application.
I was processing some recent shots of the Kittiwake wreck the other day and I noticed that the image quality wasn’t as nice as usual (most obvious as Chromatic Aberrations) and then I remembered that I had used the 8-15mm that day, rather that the RS13 or WACP I normally use for my wide angle. But of course the images are still very nice. It is a small difference - but when you know how it can be, you do notice it every time!
But the main reason for using the WACP is that it has a totally different angle of coverage than the 8-15mm. The 8-15mm is 180˚ corner to corner @ 15mm. The WACP covers 130˚-57˚ corner to corner with the 28-70mm - making it a super versatile option. But I still always travel with a fisheye too - for shooting the biggest subjects (wrecks, scenery, very big animals, etc). I take the RS13mm when splits are not important and the 8-15mm when I expect to shoot some splits or want to use the 16-35mm too (they use the same dome).