Water prevents dehydration.

If you are thirsty, your cells are already dehydrated. A dry mouth is the last outward sign of dehydration. Thirst does not develop until body fluids are depleted well bellow levels required for optimal functioning. Monitor your urine to make sure you are not dehydrated:

A hydrated body produces clear, colorless urine. A somewhat dehydrated body produces yellow urine. A severely dehydrated body produces orange or dark-colored urine.
A mere 2% drop in our body's water supply can trigger signs of dehydration: fuzzy short-term memory, trouble with basic math, and difficulty focusing on smaller printer, such as a computer screen. Mild dehydration is also one of the most common causes of daytime fatigue and headaches.