Fish, like people, need oxygen to live. A fish out of water is a fish out of its element. A fish comes fully equipped with a pair of gills, which it uses to breathe under water. The gills extract life-sustaining oxygen from the hydrogen in the water molecules, in order to regulate the amount of oxygen intake. This maintains the necessary balance of the two components of water for the fish to survive.
When a fish is taken out of water, and exposed only to air, not to oxygen and hydrogen containing water, its gills are unable to control the oxygen intake, the delicate balance cannot be maintained, and the gills inhale a lethal overdose of oxygen. The fish essentially experiences death by "drowning."