Thanks Mark Twain for the detailed
I looked up the specs ...of A7II's housing. The weight of 2.xKg is very close to that for any FF dSLR and I wonder if there is any real advantage of bringing A7II for still image shooting
Well, don't underestimate the difference in weight and size that adds up over all the items. For example, the housing for the A7 (Nauticam) weighs 2.4 kg, the Nauticam housing for the Nikon D810 weighs 2.9 kg.
Sony A7 housing dimension with handles: 320mm (w) x 170mm (h) x 110mm (d)
Nikon D810 housing dimension with handles: 350mm (w) x 298mm (h) x 134mm (d)
(Source: Nauticam USA)
Furthermore the Sony A7 weighs 474g versus the 980g of the Nikon D810. So alone housing and cam weight difference adds up to roughly one kg. I guess lenses for the A7 are also slightly lighter as most of them are rather slow (f/4).
For dives where I can get by car I don't really mind for weight and size although having a lighter gear would be better during the dive; but adhering to airline's luggage weight restrictions is always a struggle for me, so every gram counts ;-)
What really surprises me is that the depth of the D810's housing is only 2.5 cm greater while being ~13 cm (!) higher. Intuitively I'd have said that the housing of the D810 is a bit less high and wide but considerably less deep.
What I am wondering is how the A7's AF performs (I own a A7 but I do not have a single AF lens, only old Minolta MD/MC), also during night dives (in contrast to the A7R and A7S the A7/II has on sensor PDAF).