Question

We cannot breathe in water but fish can easily breathe in water why

Answer 1

We cannot breathe in water but fish can easily breath in water because fish have gills but human beings have lungs. Gills help fishes to breath in water.

Answer 2

We cannot breathe in water because human lungs are not capable of absorbing the dissolved oxygen present in water and we can breathe only through the atmosphere but since fishes have a special organ known as gill they are able to extract oxygen from the water and are able to breathe the oxygen easily in water.
In a nutshell the fishes have evolved in such a way that they can breathe in water while humans have lost their ability to breathe in water after years of evolution.