Well, for better or worse, I am a Nikon guy. I still own my original Nikon F2 that I bought in the 1970's (it is not pretty, but it still works). My oldest Nikon lens is a Nikkor that was made in the mid-1960's (the aperture ring was upgraded and the lens will work with on my digital bodies in manual mode). When I attempted to get into underwater photography in the late-1980's, I used two Nikonos II's and a Nikonos III. I still have the Nikonos cameras, maintain the bodies and occasionally shoot underwater with them.
Based on demoing the D850 some months ago, I believe you made a good choice. Despite the rumors about Nikon going out of business (I hear that a lot from folks who shoot Canon cameras), the D850 is an incredible camera with a lot of features that aren't applicable to underwater photography. Pair it with a great Nikkor lens, a good housing and decent strobes, and you'll have a great underwater camera rig. Like I said in an earlier post, I plan to purchase one in July. Yeah, I even know what lens I will be using with the camera when it makes its first trip underwater.
My oldest underwater housing is an Ikelite for a Nikon 8008s. Ikelite no longer services the housing due to its age and I may have the last o-ring sets for the housing that they had in stock. Like my Nikonos cameras, the housed 8008s still makes underwater trips.
My primary underwater rig is a Nikon D7200 housed in an Ikelite housing. I bought the Ikelite because I wanted to use two strobes and my budget was limited at the time. The housing has worked well for me.
I have shot underwater photographs with other photographers who use Subal, Aquatica, Nauticam, Sea & Sea, and Ikelite housings. I have not heard much in the way of complaints from any of them. All of the companies seem to make pretty good products. While I am sure there are others out there who may have a different perspective about flooding, most of the flooding issues I have heard about are the result of user error. Regardless of manufacturer, if you are careless when assembling the rig or skip doing maintenance on it, it will eventually leak.
I did talk to a underwater photographer who had a Nikon D200 housed in an Ikelite housing displayed in his gallery. He made some negative comments about the Ikelite housing flooding on him. When I asked him why the housing flooded, he mumbled something about having a dirty o-ring and the housing flooding as a result. I thought, "Geez, you did not maintain your o-ring and it is Ikelite's fault... go figure."
Maybe the best advice is to encourage you to visit some housing dealers and look at the available D850 options. Do some hands on with different housings and see which one will best suit you. Unless you buy and Ikelite housing, other manfacturers' housings will rival your D850 in price. You are dropping a lot of money on a housing; wrap your hands around some of the options and decide which one will work the best for you.
-AZTinman
