Hi everyone, I’m thinking about getting a Canon cinema camera for underwater video work and would love to hear your experiences. Has anyone used the Canon C80, C70, or C400 underwater, especially in blue water and when filming larger marine animals like sharks, whales, or big fish? I’m curious about real-world video quality, autofocus performance on moving animals, low light behavior, underwater housing options, and which of these cameras you would recommend for serious underwater cinematography. If you were investing in a dedicated underwater cinema setup, which one would you choose and why? I’d appreciate any advice or experiences. Thanks!”

Can't help as I've never used a cinecam underwater (and probably never will...😅), but if you're interested there's a nice test reel from 2021 by Alex Del Olmo on the C70

As for AF, for such large animals you'd probably be hyperfocal most of the time, so i wouldn't worry too much about that. MF (with backbutton focus AF to refocus) is safer to use.
Canon has stellar manual white balancing and a very pleasing colour rendering, so can't really go wrong.