Water is essential to fat metabolism and muscle building.  It helps to fight the battle of the bulge by keeping fat cells hydrated and pliable; thus allowing for an easy energy exchange, as opposed to dehydrated, rubbery fat cells that are resistant to being used as fuel.

Hydrated muscle cells respond better to workouts with improved functionality, greater contractions, and an improved ability to accept nutrients for repair, growth, and maintenance.  

The most common method of calibrating your recommended water intake is to drink at least 1/2 your body weight (in ounces) each day, more when you are particularly active.  Here is a quick reminder from Medical News Today concerning fluid intake.

For a healthy adult, fluid intake is probably adequate when one rarely experiences thirst and when urine is colorless or slightly yellow. As adults get older, they may experience less thirst, so it's advised to drink before thirst sets in. Signs of inadequate fluid intake may include dry mouth, headache, light-headedness, little or no urination, and constipation.

And remember; never leave home without water and if you think you are hungry, try drinking a glass of water because you might find out that you were not hungry at all, simply thirsty.