As you know, the human body needs water to survive.
Beyond survival it's important for many different reasons such as:
Digesting food
Transporting nutrients
Eliminating waste
Increasing athletic performance and endurance
At Poehlmann Fitness we advise that you aim to drink .55oz of water per pound of body weight each day. Some days you'll drink more due to exercise, too. Listen to your body, and drink water when you feel your body needs it.