It would be because it is the opposite of how we breathe in the surface and in land. We humans, can breathe in air, although, if we arrive at the ocean or the sea. The water traps oxygen, closed a bit tightly. Like in space, we cannot breathe because it also traps oxygen leaving it into other planets, mainly earth. Fishes, has gills for breathing. Those air trapped in the water are breathed by the fish through its gills. When, it arrives in the surface. The air or surface instead is the one to trap the oxygen breathed by the humans.