I thought my underwater photos were decent until I went to the Digital Shootout and saw what others were doing, daily. Caused me to try to up my game. I was shooting a D850 underwater with 230mm dome and 16-35. Or macro with 105vr.
I've rarely been happy with my wide-angle stuff, but I think that a good deal of that is my choice of subject matter, which is often seascapes. Unless I have a) bright sun, b) clear water, and c) relatively shallow depth, I just don't have much 'luck' in the results. Macro is far simpler and easier to emphasize a subject and light it. Don't need good ambient conditions there.
I'm mostly struggling there in both post-processing (mostly white balance) and composition, but trying to improve in both. But I'm not struggling in camera settings particularly, and it would be interesting to hear where you are having issues.
I see some indications: particulate matter in the water (really sucks for wide angle), fast currents (can be fun in video, but yes, does make it hard to put the camera where you want it when it needs to be there), and fast critters. Well fast critters are always going to be tough, unless one of the camera makers comes up with good subject detection for underwater animals like fish. That's part of the challenge and part of the reward when you get lucky. And persistence tends to lead to more luck.
When doing 'general' wide angle, (unpredictable but quickly over situations) I think the most acceptable no-time-to-change-settings wide-angle setup for me was to set shutter speed (1/160th) and aperture (F9-ish), turn on Auto ISO and then carefully manage strobe power manually based on distance of strobe to subject and ISO involved. So an ISO 800 shot might care of the ambient, and a kiss of strobe power lights the subject but doesn't overwhelm it and still allows a good color white balance since the camera was really set for ambient. Putting in strobe power at very low settings didn't overwhelm the ambient white balance, and once the strobes were set, it was really just a matter of having another subject about the same distance from the camera. That allowed me to focus more on composition and focus. Since wide angle has a pretty good depth of field, prefocusing on a reasonable distance can allow you get shots of fast moving critters, though it is often hit-or-miss there.
Some things underwater are a volume game, even for the most experienced. I find there is no substitute for dive time underwater. And since strobe position is so important, it is important for you to master getting the flash power where you want it at the exact strength you want it. You simply need to take time to get the strobes right for the subject. That said, if the subject can be repeated, you don't need to change the strobes and can then cruise around getting images easily. But you'll have to chimp the results at first to get those strobes right. That won't change with a different camera. But if you have a pool and some underwater subjects, you could practice at home.
start out taking images of things that literally don't move (like coral). or move very slowly.
