The D810 has zebras...(but no peaking). You could add an external monitor like the SmallHD which will give you peaking, false color etc. if you wish.
I would avoid using the 10-17mm in any form with the D810. The D800/D810 will out resolve it significantly. In addition, you can only use it around 16mm without vignetting, so you gain no advantage over the Sigma. The Sigma 15mm is great!
I have yet to find an EVF that is as good as an optical VF. For focus critical applications, this is important.
Lenses are the biggest single issue (for me) with Sony mirrorless. The lack of a decent fisheye is a huge negative for underwater use. This is, for me, a deal breaker.
The Sony 90mm macro is a great lens and pretty much sorts out the fish portrait issue.
For stills, the D810's AF is quicker and more accurate then the Sony's. This may not be the case for video, although I don't use AF for video underwater anyway!
The D810 is massively more bulks and cumbersome. It is also, in my opinion, still easier to use and will produce stunning still images!
Nice to have these choices!