Stick with 5d3 or switch to A7R3

5d3 with 16-35 2.8L is my current underwater setup. It's the only place it gets used.

A7R3 is my primary land camera. Canon 1Dx for other stuff.

Question is, do I sell the 5d3 and housing/dome port and get a housing for the Sony? I'm thinking I don't need 3 cameras.....

I'm not sure if it's possible to use the Sony with the Canon lens underwater as I'd need an extension that takes into account the Canon lens plus the Metabones adapter. To be honest I haven't looked, I'm not even sure where I'd need to start to figure out if it's feasible. The alternative is to pick up a Sony lens, either the 16-35 2.8, which I'm not super keen on as the entire front element moves, or the 12-24 4.0, which may be too wide for a large dome.

I could still probably get a couple grand for the 5d3 and S&S housing/dome, although I doubt I could pay for the Sony equivalents straight out with the sale, even with extras.

I'm not really into macro so sticking with wide-angle for now, shallow reef or caves are the most likely subjects.

Any thoughts or suggestions?

It is possible to use the Canon lens, but if you want to use a rectilinear lens it will be better to go with a native Sony lens. You will have better reliability with AF if you go native.

Have a look at the Sony Zeiss 16-35mm f4, it's nearly $1000 cheaper than the f2.8 and both lenses are pretty much the same optically. The extra stop in light doesn't give too many benefits underwater.

I've been really happy with the performance of my Canon 16-35 with the metabones adapter on my Sony in terms of AF reliability, etc. in some pretty challenging land conditions. I got the adapter on a whim expecting pretty horrible results but it seems to be as functional as on my 5d3 and 1Dx. You say it's possible to use on the Sony, I guess I just need to do a little more in-depth research looking for extension tube compatibility.

If it will work, I might as well go with the Sony 12-24 and just grab another extension tube for the 16-35 w/Metabones and see which I like better.

I'm using the Canon 8-15 mm on a Sony A7rIII via a Metabones V. Works well. Just a note, though: the 100 mm (Sony) to 120 mm (Canon) port adapter, plus the Metabones, adds a fair amount of weight, bulk and expense. Potentially something to factor into your decision-making.

I love my Sony a7r iii, dove with it at least 150 dives past 2 months and really liked it. Only had problems with focusing on Sony 90mm f2.8 macro, which was doable but slow and at times not reliable. Other than that the dynamic range is amazing, battery life has been improved (I would not recommend older a7 ii and a7 ii as you would need to replace battery and open your housing after every dive), camera is very compact, controls are intuitive, menu is ok. I shot with Canon before and prefer Sony workflow. One drawback of Sony vs. Canon is cost of the lenses and limited Sony lens availability vs. Canon's line-up. The workaround for this is Metabones adapter. Hope this helps.

You should also check the ID of the port system you are using - for example Nauticam uses N100 ports for SONY which seem to have an ID around 88-90mm while the Canon housings use the N120 ports which are much bigger, you want to check if you can insert lens already mounted on camera from the back of the housing. Another issue might be a zoom gear - there are gears to use the Canon 8-15 fisheye with a Metabones in some systems but probably not for the 16-35 as SOny already has a good 16-35 in f4 and f2.8, would likely need a custom printed gear??. You would also need to guesstimate the right extension.

Nauticam's port chart lists the 35.5mm N100-N120 adapter with focus knob and refers you to the N120 port chart for port recommendations. Presumably this would allow you to the Canon zoom gear and the knob on the adapter - you would need to check that out and confirm. What you could do is likely to vary between different housing options. Here's the Nauticam port charts: https://drive.google.com/file/d/1TpEOQzvfyr8lHvMXJhGoujO34sIzUjLN/view

If you went with a Sony lens, the 16-35 f4 is likely a better option, lighter and you'll never use the extra stop underwater.