Early impressions - I am finding the A7RV a very easy camera to like. Once you get used to Sony-speak it is an easy camera to set up and shoot. EVFs are not optical viewfinders. But the A7RV has the best EVF I have used so far (had R5 and A7RIV in the pool yesterday). Just as an SLR viewfinder had negatives and positives when you first tried it, compared to the Nikonos 15mm viewfinder, I think that EVFs need to be thought about as a different solution with obvious negatives and positives compared to an SLR viewfinder. Have 3 Sony users on workshop - 2 with A7RIVs - and they are all considering the upgrade.
The AF on the Sony is the best I have used underwater. I have shot older Sony's and Nikons and the R5, but not A1 or Z9 underwater. For wide angle it just works (but then so do most modern cameras). It is excellent at tracking fish with macro lens. It is the first mirrorless I have used that I judge better than any SLR I have used. Animal and Insect subject recognition work on fish, but more so the cameras generic subject detection (with subject recognition turned off) is just great at locking on to the subject and tracking it. The AF is a little slow to get going sometimes, but once up and running it is hugely impressive. It didn't work as well in vey dark places, deep under overhangs, but need to do more tests. To date favourite modes are AF-C with WIDE for wide, and Zone for macro. For macro I have used with and without subject recognition.
The Sony lens choice for UW is still lacking and is the biggest weakness currently of considering Sony. Needs a native fisheye. Canon fisheye on Metabones works well, but always feels like a temporary solution. Native would surely squeeze out more from the image quality. Macro lens range needs expanding and updating - UW photography always demands more than one focal length of macro (for my SLR I brought 60mm, 105mm and 150mm on this trip). I would love to see a new generation 60mm and 120mm to compliment the 90mm. The 90mm is impressively sharp, though. The 28-60mm is a mini-marvel with the WWL-1B and WACP-1. I have only tried the former so far (pool tests show corners perfect at f/9, becoming limiting at f/6.3 for my standards, unless in blue water). I will do a comparison test of the WACP-1 during next week's pool session. With better lens choice the Sony system would be hard to fault.
Image quality is excellent. I struggle to get a WB out of camera that I feel is perfect. I am experimenting both with using AUTO and a K WB. Obviously can be tweaked in post, but I'd rather the camera got it closer. Also I am still not quite happy with how the images look on the camera - they look noticeably better when you download them. Look a bit "compact" when reviewed on camera. Maybe I need to play with JPG settings.
I am currently using Nauticam trigger, and have a max synch speed of 1/200th, which is a limit. I have an UW Technics one to fit and HSS flashes too, but I want to give the 1/200th limit a fair test, to see how limiting it really is, once I am used to it. For now I am just going to drop my ISO by a couple of clicks and turn my strobes up.