Water makes up approximately 60% of our body weight.  However, much of that is lost through excessive exercise, diarrhea, urination, perspiration, bleeding, exhaled air etc. etc.  That is the reason we are required to drink two to four liters a day to substitute for that loss.
The intake of water is regulated by the body which provides signals in different ways: 
Thirst is felt in the mouth by a dry feeling resulting from decreased saliva. In the case of severe diarrhea, perspiration, excessive urination etc. saliva will cease to be released.        
When water intake drops, the hypothalamic cells stimulate the release of an antidiuretic hormone to store body water by decreasing the production of urine, causing water retention.  When water intake returns to normal, the same cells reduce release of the antidiuretic hormone and water retention is relieved.
The pituitary glands, located at the center of the brain, as well as renal functions, regulate plasma volume by conserving or releasing water as necessary.
Severe dieting or forced starvation and a carbohydrate-restricted diet cause severe water loss of up to 1.5 liters.
Heat Exhaustion & Dehydration
With the coming of each summer, we run the risk of heat exhaustion (loss of body salts through perspiration) when we push ourselves beyond safe limits in the sun.  Unfortunately the result is not just a tan, but also excessive loss of body fluids (dehydration) and rise in body temperature.
Dehydration occurs when your body loses too much fluid.  AND, when that happens, your muscles begin to get tired and you may begin to feel faint. AND THEN, if dehydration is severe, and there is no longer enough fluid in your body to get blood to your organs, you may begin to go into SHOCK!!! While heat exhaustion and dehydration occur at any age, they are most dangerous to babies, small children and older adults.

Dehydration in babies & young Children: This group is more at risk because:

-A large portion of their body consists of water
-Children have a higher metabolic rate
-Children’s kidneys do not conserve water as well as adults
-They have an immature immune system
-Children will stop eating or drinking when not feeling well
-They depend on others for food and fluids

Dehydration in Older Adults: This group is more at risk because:

-Have a decreased sensation of thirst
-May have inefficient kidneys
-May have medical problems such as arthritis which impairs their ability to hold a glass of fluid or even get up to get it.
-May have debilitating mind diseases which make it difficult to communicate their needs
-May take medication that increases risk
-May not afford adequate diets
-May suffer incontinence and therefore limit their intake of fluids purposely

Early Warning Signs for both Groups:

-Dry mouth
-Sticky saliva
-Reduced urine output
-Dark yellow urine
-When recognized, mild to moderate dehydration can be home-corrected

Newborns & Babies through 1 year of age:

Breast –fed or bottle-fed babies to be fed at shorter intervals to replace lost fluids
Use an oral rehydration solution if signs are mild to moderate.  (The amount needed depends on baby’s weight and degree of dehydration).  The solution may be administered in a dropper, spoon or bottle
If baby has been  started on food, provide rice cereal, strained bananas and mashed potatoes (if he baby has had them before).

Children  Ages 1-11:

Give oral rehydration solution, diluted orange juice or plain water.  (Food can also replace lost fluids)
Encourage extra fluid intake, (children between 4-10 years should drink at least 6-10 flasses of fluids) or sucking on Popsicles
Provide cereal mixed with milk or water

Children Age 12 and Adults:

Stop their activity and have them rest
Bring them out of the direct sunlight into the shade or air conditioned area.
Elevate their feet
Remove all unnecessary clothing
To children, administer a rehydration drink, water or juice.  (Provide 1.9 liters of cool liquids over 2-4 hours)
To adults, administer at least 10 glasses of liquid
Provide more fruit and vegetable (for water content & salt balance)
Keep them rested and continue fluid replacement for 24 hours.
Total rehydration with oral fluids usually takes 36 hours, but in most cases, people begin to feel better within a few hours.
In case of home treatment of dehydration continue to watch for:
Decreased alertness
Dizziness, lightheadedness or feelings of faintness when rising from lying or from sitting
Decreased urination
Increased or more frequent signs

Water and dieting

Water is a natural appetite suppressant which contains zero calories!!! Our bodies need it, thrive on it and actually lose weight by it.  The more water you drink, the less you will eat.  A large glass of water 15 minutes before a meal will cause you to feel full and therefore eat less.  Also, if he water is icy, it may cause a slight shrinkage in your stomach, also causing you to eat less (If you feel you are overweight, instead of drinking just 6-8 glasses of water per day, drink an additional glass for every 12 kilograms of excess weight.)
Restricting water intake will cause the body to think it is in danger of dehydration and starts immediately to store all the water that it can between the cells.  This causes swelling of the limbs and shows up on the scale as EXTRA WEIGHT!!! On the other hand, if you do drink sufficient water, the body will release that stored water.  The new will flush out the old.
Actually, you need water for your body to burn fat:
Without water your kidneys do not function well
The liver must step in to help
While the liver is helping the kidney do its job, it cannot burn as much fat as necessary for use as fuel in its original task, therefore that fat gets stored in your body.