By drinking water, you maintain your body's ability to deliver oxygen and nutrients to the working muscles and all other cells.
Water also carries away the waste products created by exercising muscles.
If you are dehydrated your body must begin using the water content of your blood. Blood volume can get dangerously low in order to produce sweat to protect your body from a high body temperature. Athletes lose concentration, coordination and endurance capacity when they don't replace water lost from sweat. Dehydration reduces the ability to sustain exercise for longer than a minute or two. So if you can pin your wrestling opponent in 30 seconds, you may not worry about hydration.