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).
I'd love to hear from anyone shooting a Song DSLR as to what does and doesn't work well for them.
Well, I've never had anything else but Sony so it is difficult for me to compare and I can only state what I like and what I dislike.
I own a Sony A65 and use it in an Ikelite housing with a Sigma 10mm fisheye, a Tamron 60mm Macro and the Tamron 17-50 (which I would like to replace with the Sigma 17-50 because of OS which is great for movie and silent HSM focus instead of scredriver AF). The Sigma is a great lens and the 60mm too (especially for its price tag), although I sometimes wish I had a longer macro lens (~90-105mm).
Basically I am quite happy with my A65 but there are a few things that bother me. I bought the A65 because it was the only current body of Sony for which a housing was available that time. The A77 is a much better camera in terms of handling and functionality. AF of the A65 does not leave much to be desired and so does image quality (although as I also own the A7 I miss the dynamic range and ISO performance). What I am really missing when doing macro is DMF so all I can do is using lock-on AF and then focus by moving the camera back and forward.
What is really great is the live view which gives an instant impression of what the picture will look like (except DOF). I never use the viewfinder underwater but the viewfinder is also quite good in terms of size and brightness. Newer electronic viewfinders have a faster reaction but for underwater the A65's liveview is more than sufficient. One of the advantages of the A65 is a rather small body size, but this advantage is offset by the rather big size of the Ikelite housing. Also the smaller body leads to less direct access buttons.
Finally I have to say that there are some functionality disadvantages but those are model and not brand related. But all at all you can get great results.
If I had to decide now which equipment/brand I would choose I would definitely stay with Sony but not the A-mount cameras. Instead I would choose E-mount (A7 or A6000, depending on the budget ;-)), inspite of some missing lenses (but Sony keeps releasing lenses and so will 3rd party manufactures like Sigma do). If someone wants to go A-mount for whatever reasons I would choose at least the A77II, better the A99 (I think there are housings available, please correct me if wrong)