I've used a D800 for years now and have also shot the A7II in a Sea & Sea housing with a flash trigger.
I think while the housing size is a bit smaller, it's not all that much difference in the water. You have to use a port adapter, extension and mid-sized DSLR sized ports on the a7II. So what are you really saving?
I mostly use a Zen 170mm dome on the D800 with a Sigma 15, or even a 17-35mm if I don't care too much about the corners (blue water). Those are pretty much the same sized ports on the a7II.
As far as using Canon lenses on the a7II, it gets pretty awkward and the AF is even slower... There is a new Zeiss Batis 18mm lens that shows good promise. But all of the Sony lenses are expensive, the Sigma 15mm FE is like $600.
I didn't have battery problems on the a7II, but I mostly shot stills. Video is more of a problem. I do like that I can basically shoot all day with the D800 on one battery.
The viewfinder is one area that I'd also give a huge benefit to the D800. An optical viewfinder is so much brighter and easier to use than an EVF. Yes, you give up focus peaking and some of the other more electronic features that are nice. But again those are more useful for video. And of course the better, faster AF on the D800. I found that the a7II sort of jumps a bit for macro, manual focus is pretty much necessary.
The a7II is lighter. And my older D800 Nauticam isn't the lightest for sure. But I'm sure that the new D850 housings will be lighter as they will be a few gens newer.
I've also tried the Oly E-M1 MKII and D500 for something smaller, but I keep coming back to the dynamic range and quality of the D800, although I think if I was shopping right now, I'd buy a D500, I continue to be impressed with it.
I've some articles on these comparisons on my blog:
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