Some nutritionists state that 80% of people are walking around dehydrated. Water is pure liquid refreshment and accounts for a large percentage of what makes each of us "human". Almost 2/3rds of our body weight is "water weight":

Blood is 83% water
Muscles are 75% water
The brain is 74% water
Bone is 22% water

Water is necessary for your body to digest and absorb vitamins and nutrients. It also detoxifies the liver and kidneys, and carries away waste from the body. If you're dehydrated, your blood is literally thicker, and your body has to work much harder to cause it to circulate. As a result, the brain becomes less active, it's hard to concentrate, your body feels fatigued.

Water - The Miracle Worker
Simple water -- when it's pure and free of contaminants -- is truly a "wonder drug." Without chemicals, additives, or anything unnatural, a steady dose of 8 glasses of water a day (ideally 1⁄2 your body weight in ounces of water) will:
Improve Your Energy
Increase Your Mental and Physical Performance
Remove Toxins & Waste Products from your body
Keep Skin Healthy and Glowing
Help You Lose Weight
Reduce Headaches and Dizziness
Allow for proper Digestion
Help to keep you more Alkaline

Drinking Water is Heart-Healthy
The findings of a six-year study of more than 20,000 healthy men and women aged 38-100 in the May 1, 2002 American Journal of Epidemiology found that women who drank more than five glasses of water a day were 41% less likely to die from a heart attack during the study period than those who drank less than two glasses. The protective effect of water was even greater in men.

Drinking Water and Weight Loss
Water is a natural appetite suppressant, so developing a good water drinking habit can be a long-term aid in achieving and maintaining a healthy weight. Doctor F. Batmanghelidj MD, author of "Your Body's Many Cries For Water" says most times your “hunger” is your body asking for water – not food.
It's also important to remember that when the body is dehydrated, fat cells get "rubbery" and cannot be easily metabolized. This means that it's harder to lose when you don't drink your water.

Who Can Benefit From A Water Regimen?
Water is especially important for pregnant women and nursing mothers. For athletes and work-out fanatics, drinking water reduces cardiovascular stress and improves performance. And, since water reduces body temperature, it makes the whole exercise process safer and more effective.
Water is also an important "healing tool" for people with a history of kidney stones. Since water dissolves calcium in the urine, downing at least 8 glasses daily reduces the risk of stone formation. Drinking water is also valuable in preventing urinary tract infections in both men and for women, flushing impurities out of the system.
Even mild dehydration makes you more susceptible to viruses.