The human body is used to the conditions on land. Even when he dips into water, to wash or to swim, he still behaves as a "land animal": He holds his head out of the water as much as possible, doesn't stray from the beach and so on. Diving forces man to acclimatize to completely different conditions from those known to him. A diver that succeeds in understanding the new environment, adjusts to it and obeys its rules, will quickly find that he feels "at home" no less – and perhaps even more so – than on land.

Breathing:
Human beings, like most animals, need a continuous supply of oxygen in order to exist. The body cannot exist without oxygen, and lack of oxygen will quickly cause loss of consciousness and drowning. We absorb the oxygen we need by breathing air from the atmosphere, which contains the amount of oxygen we need. When diving underwater, we cannot breathe from the atmosphere, which is why we need a continuous supply of air using S.C.U.B.A or a tube ("snorkel").