@ChrisRoss that worked! I got lucky on the first dive as the flash didn't actually fire due to the connector to the bulkhead not set properly (another reason I might get the A1 seafrogs and use fibre optic... The sync connector on the seafrogs is a pain to align). Had the flash fired, at 1/500, half the image would have illuminated destroying all the photographs. The mirror top really helped and I tried out everything to make sure before going in.
All I can say is WOW. The strobe makes such a huge difference in terms of colour, vibrance, and everything else. I'll post some photos I took when I'm back in London, and have had time to process the work. With the strobe, the shots look amazing straight out of camera, compared to ones without which will need a lot of editing.
Also sort of glad I ended up with the ys-d3. The fast cycle times really helped in certain circumstances. Although, at times I did get trigger happy. The strobe kept up for 10 shots but then "rested" for 90 seconds before replenishing.
I'll experiment with the strobe some more on my next trip. But would you say getting another strobe and running at 11 each (instead of one at 22) would give better results? It'll definitely cycle a lot faster. But I expect if not set up properly, the backscatter might be more difficult to manage. Might even try iso 400 and strobe at 11, or even higher ISO. On land, I can comfortably push the A7iii to 6400 ISO with denoising in post.