I've always felt that it was hard to get close to big animals like sharks and that more reach is required over something like a mid range zoom.
Alex had talked about using a 60mm lens behind a dome for sharks and on a recent shark dive in Yap, I decided to try my old AF-D 60mm macro Nikkor on my D850 behind a Zen 170 dome and 70mm extension. I could of used a smaller extension, but it was what I had.
The results were quite stunning, I felt. You do need clear conditions to have it work out though, but AF was fast and I had lots of range. The longer reach shortens up the background too.
I also switched back and forth from FX to DX mode to see how that felt. I think it's probably more valuable for macro work with the 60 to get more DOF and will try that next week in Palau.