Water is a regulatory food. It is essential fer regulating metabolic activities.
Water is an inorganic constituent of food, but it is not a food. Neither it provides energy nor it builds the body like protein. But it is essential for life. Water is the main constituent of protoplasm, body fluids, blood, lymph and tissue fluid. It forms about 90 percent of the fact Two-third of human body weight is water.
An individual can live without food for 1 week but he cannot live without water for 1 day.So water is an essential part of our life. 
1 5 1
The existence of liquid water, and to a lesser extent its gaseous and solid forms, on Earth is vital to the existence of life on Earth as we know it. The Earth is located in the habitable zone of the solar system; if it were slightly closer to or further from the Sun (about 5%, or about 8 million kilometres), the conditions which allow the three forms to be present simultaneously would be far less likely to exist.[14]
Earth's mass allows gravity to hold an atmosphere. Water vapor and carbon dioxide in the atmosphere provide a greenhouse effect which helps maintain a relatively steady surface temperature. If Earth were smaller, a thinner atmosphere would cause temperature extremes preventing the accumulation of water except in polar ice caps (as on Mars).
It has been proposed that life itself may maintain the conditions that have allowed its continued existence. The surface temperature of Earth has been relatively constant through geologic time despite varying levels of incoming solar radiation (insolation), indicating that a dynamic process governs Earth's temperature via a combination of greenhouse gases and surface or atmospheric albedo. This proposal is known as the Gaia hypothesis.
The state of water also depends on a planet's gravity. If a planet is sufficiently massive, the water on it may be solid even at high temperatures, because of the high pressure caused by gravity.
There are various theories about origin of water on Earth. Water is the medium of life. It is the most abundant compound found in all organisms. It varies from 65 to 89 % in different organisms. Human tissues contain about 20 % water in bone cells and 85% in brain cells. Almost all the reactions of a cell occur in the presence of water.