The question of how much drinking water is needed for good health is a complicated one.  In reality the question should be how much fluid do we need as the Mayo clinic noted: 

 

Although the "8 by 8" rule isn't supported by hard evidence, it remains popular because it's easy to remember.  Just keep in mind that the rule should be reframed as: "Drink at least eight 8-ounce glasses of fluid a day," because all fluids count toward the daily total.http://www.mayoclinic.com/health/water/NU00283

The proper amount of fluid required varies depending upon a number of factors.  The most common are climate, disease conditions including fever or fluid loss, and strenuous work and athletic activity.  As nontechnical as this sounds, the body has an excellent feedback system and most experts suggest a total fluid intake around 6-8 glasses and to simply drink when you are thirsty.  Drinks with diuretic properties like coffee, tea, and alcohol may increase the need for other fluids. 

Giving patients advice to drink 8 glasses of water rather than fluids a day can actually have harmful effects.   Individuals who already consume large amounts of fluids (from fruits, vegetables, etc.) have no need of adding an additional 8 glasses of water.  Most people are not aware that drinking excessive water can have serious adverse health effects and in rare cases result in coma and death.  The abstracts below are examples from the literature that document some of the more severe cases of water intoxication. 

Note:    These mini-reviews are designed as updates and direct the reader to the full text of current research.  The abstracts presented here are no substitute for reading and critically reviewing the full text of the original research.  Where permitted we will direct the reader to that full text.