Water is important for life on the earth. It is required for all life processes, such as, cell growth, protein formation, photosynthesis and, absorption of material by plants and animals. There are some living organisms, which can survive without air but none can survive without water.
All the water present on the earth makes up the hydrosphere. The water in its liquid state as in rivers, lakes, wells, springs, seas and oceans; in its solid state, in the form of ice and snow, though in its gaseous state the water vapour is a constituent of atmosphere yet it also forms a part of the hydrosphere. Oceans are the largest water bodies in the hydrosphere.
Our earth is the only planet in the solar system which has water in abundance, hence, it is often called a ‘Watery Planet’. About 71 % of the earth’s surface is covered by water.
Oceans form a single, large, continuous body of water encircling all the landmass of the earth. They account for four- fifth of the Southern Hemisphere and three fifth of the Northern Hemisphere. They contain 97.2 % of the world’s total water.
There are four principal oceans in the world which are separated largely on the basis of their geographical locations. These are the Pacific Ocean, the Indian ocean, the Atlantic Ocean and the Arctic Ocean.
All the other seas, inland seas or the arms of the oceans, are counted within these four main oceans.
Bibliography : NIOS Geography Book
Reference : http://www.nios.ac.in