Not all fish can breath underwater interestingly enough, meaning if they can’t reach air they will drown. The first example that comes to mind are lungfish, but there are others, such as arapima. I think bettas also are obligate air breathers, but I might be misremembering.
I’m not an ichtyologist, but iirc most fish can breath without moving. Some sharks must move to breath, but plenty of them don’t have to, i.e. the wobbegongs