it is necessary because of the following reasons:
It makes up well over half of your body by weight.
It is crucial for transporting oxygen through the body (the blood's oxygen carrying red cells need the water in the blood to move) blood is 95% water.
It is needed for the digestive system
Each cell has water in it.
The lungs need water for the mucus membranes to trap tiny particles in the air and prevent irritation of the lungs
another poster mentioned cooling the body via perspiration and removal of waste (water carries waste in the form of urine)- excellent points.
plz mark it best