Harvard School of Public Health (HSPH) established a "Healthy Beverage Guidelines" page under The Nutrition Source.

In concern to beverages, Water is considered Level 1. HSPH stated the following,

"Water provides everything the body needs—pure H2O—to restore fluids lost through metabolism, breathing, sweating, and the removal of waste. It’s the perfect beverage for quenching thirst and rehydrating your system. When it comes from the tap, it costs a fraction of a penny per glass. Water should be the beverage you turn to most of the time."

Water intake requirements differ from person to person but HSPH states women should take in about 91 ounces (11 cups) while men should take in 125 ounces (15 cups) a day.