I've had my D850 since March 2nd and have yet to even open the box. It has sat in my camera safe (gun safe haha) untouched for 5 months plus.
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Seriously, I do think the D500 is a better camera for UW for a couple reasons. I'd still go for the D850 myself for certain reasons. Currently shooting D810 UW and love it.
If you're stuck with an extra D850 and nothing to do with it, I'd be happy to help you out there!