I've not used the Canon systems, but I do have a Nikon D500 that replaced my D7000 (hobby enthusiast, sports and bird photographer). I've got plenty of lenses and shoot raw. The new D850 is a sweet piece of kit, but it wasn't out when I picked up the D500.
That being said, I ended up choosing the Sony A6000 mirrorless system with a Meikon housing for my underwater shooting. Worked great for the last 2 years. ~25MP with 1080p(60/30/24) video and awesome low light focus (maybe not as good as the new D850 with Expeed5), manual settings, and RAW (Sony *.ARW) files. Also has a setting for underwater white balance that works really well on its own and even better when used with a red filter (67mm wet). AND you can access ALL the menu settings and zoom with the housing. The stock 16-50mm lens (while not the fastest) works great underwater with a pair of video lights; but you're stopping down anyway for DOF. There is even an updated version of the housing that has a dome port, which I don't have. I also picked up an E to F-mount adapter to use some of the Nikon glass with it when on land and in need of a more compact system.
Unfortunately, I flooded the rig down in Playa last September when I unknowingly got a piece of the desiccant stuck between the O-rings when I swapped batteries on the SI. Nothing like watching your housing fill slowly with water and killing the camera when at depth where there is nothing you can do about it; but frankly, that's the whole reason I got it. I didn't want to ruin any of the more expensive DSLRs if it ever did happen (which it did). Out $600 rather than $1800+. My wife wasn't even mad!
Just a thought. Do what you will.
As far as the large prints go, 20+ MP will get you plenty large prints, and you can always upscale if needed in Photoshop if you're doing murals. Remember that the old D1 (5.1MP) and D2x (12MP) both had plenty of resolution and pixel density for the pros back in the day. Crazy that we are at 45.7MP and growing...soon we will be at medium format resolution on prosumer models!