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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.

deep sea fish will die when brought to the surface because of the change in pressure, it  body is used to great pressure of deep water  and inside its body it is creating  apressure that is opposing the pressure of water to prevent from being squeezed and if it is brought to the shallow part lesser pressure of water would be present but the pressure of its body is still the same so there is a tendency that internal structures of its body will rapture due to greater pressure inside than outside