Various metabolic and physiological changes takes place when you workout. There in increase in core temperature of the body, this leads to perfuse sweating leading to dehydration. Lack of hydration can lead to cramps, dizziness, light headedness, faintness, pH imbalance etc. You need to consistently replace the fluids you are losing when you exercise. It won't be wrong to say that water acts as a coolant aiding in thermoregulation. But be sure that during exercise you don't drink too much water as it can lead to sensation of vomiting and abdominal discomfort. Drink sip by sip frequently and keep your body hydrated.