Here is a guideline (don't worry, the math is easy!) to keep yourself healthy and hydrated:

Start by taking your body weight in pounds -
divide that by two -
and there you have it - the minimum amount of water your body needs daily (throughout the day, not all at once).

weight = 150
divide by 2 = 75
75 ounces per day, and another 10-20 ounces or so if you are working out.
75 oz is about 2 1/4 liters.

weight = 180
divide by 2 = 90
90 ounces per day, and another 10-20 or so if you are working out.
90 oz is about 2 3/4 liters.

Drinking enough water can assist in balancing every organ and system in your body, from the immune system, and the endocrine and digestive systems, to assisting with the health of your kidneys, liver, colon, skin, memory, energy levels, weight loss, and more.

It is important to remember that this water should be spread out throughout your waking hours, and not consumed all at once.
There has been a lot of talk recently about water intoxication, which is very rare, especially in healthy adults, and more often occurs in people like marathon runners, and extreme athletes, who do not replenish their sodium/electrolytes while exercising.
The kidneys of a healthy adult can process up to 15 liters of water per day - this isn't recommended, but you can see how much it would take to overload the kidneys.