Your body likes to breathe. Since you didn't google the answer, I'm not either, and keeping it short for a change.
Wellllllll. Y'see. In your throat hole(esophagus) you have a funny little flap (epiglottus) that let's food and drink go down to the stomach without getting into the lungs. When you breathe the flap opens to let air into the lungs. When you're drowning, your body needs the oxygen or it will start shutting down systems, so it forces you to inhale and open that flap. But if only water is available, water gets into your lungs instead of air, and your body shuts down....