How come fish last so much longer out of water than humans can in water?
My WAG is that they’re ectothermic (what used to be called ‘cold blooded’), which results in their having much lower metabolic rates than homeotherms (‘warm blooded’) like people. Given that oxygen is one necessary ingredient in an animal’s metabolism (in an aerobic organism - there are single-celled organisms that work differently), an animal with a slow metabolism needs much less of it than one with a faster one.
Air carries much more oxygen than water. The reason fish can’t survive in air isn’t due to lack of oxygen, it’s due to surface tension and drying causing their gills to collapse and stop working. Even so, while their gills remain wet they can get enough oxygen to survive a little while.
Water doesn’t carry anywhere near as much oxygen as air, so even if you do try to breath water, there simple isn’t enough oxygen there to keep you alive.