"AFL hold" is buried in one of the menus somewhere. Switch it to "on" and you're good to go. Point at something a couple of metres away, e.g., press AFL once, and that's it. No need to re-focus between shots, unless you want to.
Before I went on the trip, I thought that I would be using 1080, 50fps 10-bit. However, the advantage of 10-bit seems to be above all in "extreme" grading. I couldn't see any difference on any of the (not very modern) screens in my house. 4k gives you the advantage of being able to zoom in a little at least in post, and provided your shutter speed is twice the frame rate, i.e. 180 degrees, then the slo mo from slowing down 50 fps is really useful. Things like shots that just lasted 2-3 seconds in real time can be made to stand out a bit more. If you want to go to less than 42% slo mo however, then you have to use 1080. I just put these different options in my custom pre-sets, so it's easy to switch. Variable frame rate is only available with manual focus of course....
One bizarre problem I had was that the camera completely froze at the start of a dive. I had just pressed AFL. Nothing I could do underwater would unfreeze it, including turning it off, pressing any of the buttons, etc. Only after the dive when I could take the battery out and re-insert it did the camera work again. I noticed that there were loads of "this video file cannot be played" on the card in the slot I'd been using. I realised that after a few days' use and transferring my shots every day to the hard drive, the card would probably need re-formatting. It seemed to do the trick, and the problem didn't occur again.
Edited by matdiver1234, Yesterday, 08:58 AM.