If the importance of a nutrient is judged by how long we can do without it, water ranks as the most important. A person can survive only eight to ten days without water, whereas it takes weeks or even months to die from a lack of food.

Source 

 Major Function 

Quantity

 Water is replenished by drinking liquids, preferably those without caffeine or alcohol, both of which increase the output of urine and thus dehydrate the body. Many foods are also a good source of water fruits and vegetables, for instance, are 80 to 95 percent water; meats are made up of 50 percent water; and grains, such as oats and rice, can have as much as 35 percent water.

Water circulates through our blood and lymphatic system, transporting oxygen and nutrients to cells and removing wastes through urine and sweat. Water also maintains the natural balance between dissolved salts and water inside and outside of cells.  The human body is 65 percent water, and it takes an average of eight to ten cups to replenish the water our bodies lose each day. How much water a person needs depends largely on the volume of urine and sweat lost daily, and water needs are increased if a person suffers from diarrhea or vomiting or undergoes heavy physical exercise.