Thank you all for your insights!
I've been going back and forth between D500 and D850 for quite some time so all posts are of interest. Do you think there will be a shift with the release of the mirrorless Z6 and Z7?
I haven't seen any mention of housing for either of those, but then again this may be in the works. I guess the question you can ask yourself is what shift there was with the Sony A7 series for example, as they're mirrorless full frame as well.
There are full frame adaptors for the new mount with the Z6/7 I believe so you can re-use your old lenses, should they be suitable for underwater.
What I would consider would be:
* Battery life
* Lenses and compatability
* Housing availability
* Size, is it really that much smaller than a crop DSLR? possibly..?
* Heat, this isn't a big issue, but I believe mirrorless can increase fogging somewhat due to the heat they generate from powering the viewfinder/screen - but I guess if you keep your silca packs and your housing clean this isn't such an issue.
I'm sure if you looked at the mirrorless of today vs DLSRs of today, and you have your criteria, you could probably make up your mind now, assuming the Z6 and Z7 doesn't bring anything truly innovative to the table which UW photographers can benefit from.