Question

Why human beings cannot live in water?

Answer 1

Hint: Humans are terrestrial animals and are adapted to live on land. The absence of gills, hydrogen atom bound oxygen and lesser surface area of the lungs are the reasons why humans cannot live underwater.

 Complete answer: Water is needed for the survival of every living being on Earth but this simply doesn’t mean that one has to live in water to survive.
Aquatic animals from the beginning of evolution have adapted themselves to the aquatic environment in such a way that can live underwater but humans cannot.
Humans are terrestrial and live on land. Therefore, humans and fishes have different ways of getting oxygen from their respective habitats.
In humans, lungs take in oxygen and expel carbon dioxide. Fishes have a specialized organ call as 'gills' that aid in breathing underwater. Gills take in water, extract dissolved oxygen and expel carbon dioxide, humans cannot breathe oxygen dissolved.
Moreover, the oxygen present in water is bound to hydrogen atoms as compared to free oxygen in the air.
These are some reasons as to why humans cannot live in water.

Note: Not all aquatic animals breathe the dissolved oxygen. Fishes like dolphins and whales have to periodically swim up towards the surface for oxygen.