Underwater camera setup

SafariBob wrote:

i understand the strobes are important to add back in the reds.

It's not just reds; it's all the colors. If you try to color-balance a natural-light underwater photo, you'll find the water column shifting in all sorts of weird ways. With strobe-lit shots, it stays a brilliant blue (provided sufficient clarity at the time of shooting). There is also the matter of shadows - the sun is often an important piece in the composition of underwater shots, and shooting into the sun inevitably means that your foreground subjects are shadowed, meaning artificial light is required.

yes. This is the crux of the challenge as I see it. Will probably start with a 3000 lumen torch or video light.

That'd be barely adequate for macro. Shooting wide-angle, it wouldn't even register. Put it this way: a decent mid-range strobe, while firing, is about as bright as a one million lumen light. Those three thousands are the approximate equivalent of a fart in a hurricane. With an A7R IV, shooting wide-angle, anything less than a pair of Inon Z-240s is basically a waste of money, and even that is quite marginal - I'd put Inon Z-330 and Sea & Sea YS-D3 as the baseline for this kind of camera, with a distinct preference toward Retra Prime/Pro, OneUW 160X, or possibly Ikelite DS-160/161. Seacam SeaFlash 160D is great, but way too expensive, in my opinion.

edit: I will probably start with a seafrogs and an 8 inch dry dome

You will need to accept some compromises in image quality away from the center of the frame, and stop down a lot. I shoot APS-C with a SeaFrogs 8-inch dome, and with a 10-18mm lens (15-27mm equivalent), my corners are still noticeably soft at f-8-f/11. Examples:

Things get better at smaller apertures, like, say, f/13:

Or f/18:

Or all the way at f/22:

But even with powerful strobes, small apertures are rarely usable due to the need to properly expose the background without going into ridiculously slow shutter speeds or high ISOs. For you, with a 60MP full-frame camera, this effect will be considerably worse.