I have D850 since its launch to replace my D810 and just came back from a trip to Indonesia with my Nauticam D850 housing.
This is the first FF setup underwater for me though as my last rig was D7100. Worked pretty well but I still need to get a bit more
familiarized with changing from aps-c to FF. Macro is a bit of a pain as my working distance is completely changed and I will need to
change my domeport from 200mm (worked well with my 10-17, 16-35 with apsc camera) to 230mm. AF is better which is not surprising. The new natural light wb works really well with Inon strobes and wb calibration against my white fin at depth up to 10m
is very convenient and more accurate than with my d7100. May be because of bigger dof issue, I never had to adjust my AF fine tune on D7100 with my 105/2.8 VR but with d850, AF fine tune is a must. Luckily auto fine tune feature is very handy. A bit annoying that at usual macro distance, it was +7 but at further distance 3-5m, it was like -13. But for underwater stuff, it does not matter since I would be using it at near minimum focusing distance anyhow.