Canon R or Sony a9 underwater

No experience with Canon or Sony beyond their point-n-shoots.  I now shoot Nikon D850 underwater, after D810 and with a Sony RX100 for backup.

Looking at the specs, I would pick the Canon simply for the extra resolution, but I know nothing about the other camera capabilities.   If one has significantly better autofocus than the other, I would choose that one.

I will say that every time I've seen Sony's user interface (starting with RX100 II), I've been both frustrated and disappointed.   Canon's menus were far better for underwater use, at least with their point-n-shoots.

Beyond that, let's talk using full frame cameras underwater.   I can only speak from Nikon experience though.

For me, I have only three choices of lenses, unless I want to go off the reservation and try to gin up a port/extension system that works with some obscure lens.   FX cameras seem have zero choices for normal lenses.   You know, lenses in the 35mm-to-something range.  I have a choice between a few macro lenses, a couple of fisheyes, and maybe one wide zoom.   Since I don't like fisheyes, I effectively am limited to a 105mm macro and a 16-35 zoom, both with vibration reduction.

Furthermore, shooting a wide zoom requires a big dome port to get reasonable corners.  And you don't buy, carry and dive with a rig like this without the goal of getting reasonable corners.   This means B I G dome port.   If you have seen rigs like this before, let us know because you may have no idea just how ... controlling a big dome port is in terms of logistics of travel.  It's the underwater equivalent of having a 600f4 lens - woohoo! - but now you have to carry it to and from the dive resort.

Let me give you a heads up.

230mm dome port.   I had never seen one before unboxing mine.

The alternative to a big dome port?  Either fisheye, or perhaps one of the new, ultra-expensive and heavy wide wet lenses like the WACP2 (at $7500 and a dry weight of 7.06KG).

Not only that, my corners were ... poor anyway, until I added the special Sea and Sea Internal Corrective lens, made just for 16-35 lenses behind 230/240mm dome ports.

So - if one of your cameras has more lens offerings for underwater use, there's another good way to choose one or the other.