While I have no experience with it, the recent release of a diversity of cameras that can shoot full frame open gate (3:2 form) has made me wonder how this would work underwater. It would seem that by extending the corners (aka shooting in 3:2 vs 16:9) it would really make corner sharpness much worse and we would have to shoot at smaller apertures and further push our lenses and cameras (and ISO).

I'd love to hear people's experiences with this in real life, or thoughts.

I used open gate 4:3 several times with my GH5 and WWL. It works and it gives you some extra features in post, i.e. In macro static shot you can add a fake panning or just reframe. If available, I suggest to add the 16:9 reference frame on the display.