keeping yourself hydrated through out the day is one of the most important and basic things you need to remember to stay healthy and fit. here’s why:

Some conditions and climates require us to have a larger input of water than what we are accustomed to, like in hot summers or after a dehydrating exercise or job that make you sweat a lot.
If we can’t make up for the water loss from our body we tend to feel week and dizzy. If this condition is sustained long enough, we might lose consciousness, which is not healthy for the body because it indicates that some organs are malfunctioning and will soon face a lot of complications. Unconsciousness is due to hypotension, which is low blood pressure, because the lesser the blood circulates and reaches the brain, the lesser is amount of oxygen delivered there, causing headache primarily, then unconsciousness, which can lead to DEATH if sustained long enough!!

In normal conditions, the body loses 2500 ml per day of water on average through urination, sweat, feces, and insensible loses via skin and lungs. However our input is divided between drinking fluids, foods, and metabolism. Drinking fluids brings back 1500 ml of the lost 2500 ml, while foods contributes to half of that (750 ml) and metabolism 250 ml, on average.

When performing exercises for an hour and a half (90 minutes) or more, it would be advisable to drink water from time to time to keep the body hydrated to maintain the right bodily functions needed for the workout. However,massive intake of water before, during, and after exercise is not healthy. In fact,  digestive system does not function as effectively as it normally does during exercise if large amounts of water and food are in the stomach which can lead to vomiting, nausea, and in some cases bleeding from the nose.

During and after exercise the body doesn’t only lose water, it also loses electrolytes like sodium. Thus, it is more efficient to drink bevereges that contain these fluids such as Gateorade. Such drinks are most preferably consumed by endurance athletes like marathon runner, bikers, swimmers..etc

Source: Home of Sports Medicine and Exercise Physiology