4, fish can drown, the same way humans can asphyxiate. Some sharks can never stop moving as there isn't enough oxygen in the nearby water to facilitate their metabolism, so they need to continuously pump water through their gills, they stop swimming they keel over, literally.
I'm a graduated marine biologist with nits to pick.
The TL;DW is: in cladistics (which is the preferred way of classifying animals these days), there's no way to include all things that we would call "fish" without also including all mammals, reptiles, and birds. So either people are fish, or fish don't exist.