Question

why do fish die when taken out of water

Answer 1

The water acts as hydration for the fish's gills. So, when out of the water and all that air is flowing by the gills but no water, it is essentially like something is blow-drying the fishes gills. So the gills dry up and can no longer function properly, which is why they "suffocate" and die out of the water.

Answer 2

Gills are richly supplied with blood capillaries and can readily absorb the oxygen dissolved in water. ... When fishes are taken out of water, the supply of oxygen to the fishes is cut as the fishes cannot absorb and breathe using the oxygen present in the atmosphere. Hence, they die after some time