I am an amateur underwater photographer. I felt that my D800 was getting long in the tooth so I went up to mirror less - the Z7II. I got that last year. Had I got it this year, I might have gone with the z8 for the fast focus but that cost more and it was not in the market last year and no one knew when it might come on line.
First off, the housing, not the camera is the major expense. Housings have much lower production rates than cameras so that makes sense.
Secondly, I compared the Ikelite housing and system to the Nauticam. The Ikelite housing is cheaper than the Nauticam. But the system cost is pretty similar. The ports, extenders, arms, strobes, focus lights, floats are all close to the same price. Plus the Ikelite is a box and it can come out of synch and can require servicing (I used to have an Ike housing). The Nauticam housing has superior ergonomics, it stays dialed in. Also, you can get a vacuum lock system for a Nauticam housing and having one pretty much makes the system pretty close to flood proof, I have never had a problem with a housing once I get a vacuum lock on it.
Now, the advantages of a mirrorless system vs a DSLR. The D850 is still a really good camera. And the F lenses have become relatively inexpensive. The mirrorless cameras have advantages. You can focus pretty near anywhere on the field of view. The lenses tend to be lighter and have superior optics (but this comes into play pretty much only if you are pixel peaking). Also, Nikon seems an F to Z mount inexpensively so you can use all of your old F glass until you go over to mirror less.
Mirrorless has some really nice advantages. Wide angle zooms and long telephoto lenses in F tend to be big. In mirrorless, they are much lighter with better optics. I have an old zoom long F lens and I now have a z zoom long lens about the same range and the z lens is easily hand held and the F lens is about the size of a bozooka.
The problem with underwater photography is in is not cheap.
Remember, skill in photography matters more than equipment. I have a buddy who dives with an Olympus mirrrorless and it is old. My system is superior optically but his has advantages … it is far more compact making macro far easier (my system is so large that it is just not possible to line up quite a few macro shots). He can shoot one handed, I have to shoot 2 handed. He is a good photographer. A big advantage he has is versatility. With his wet lenses, he can shoot wide angle and macro. I put on a lens for the dive and I am now shooting wide angle zoom, zoom, 60 mm macro or 105 macro. On a recent dive I got really close to a feeding eagle ray and he did not care about me. So had I had my zoom lens, I could have gotten in nice and close and taken shots at 24 mm.
Years ago, I used to shoot an old canon point and shoot the G10. On the Canon corner, there was Gilligan, who shot a G8. And he took really great shots with a limited camera.
My current Z7II focuses pretty well. It is superior to my old D800. A Z8 has a focus system that is faster which would be really nice. But the Z7II works very well.
If you live close to a good reef photographer shop, go there and get your hands on the gear. I makes a difference if you can get your hands on the system and see how it works. Backscatter is good. I live in FL so I do business with Reef Photo and Video and they are really good.
Also, Backscatter has a really good website and you can watch videos and reviews of what is on offer.
