Why does it hurt to dive into a pool on your stomach?

Answer 1

Because the surface area of you stomach on chest need to displace enough water to go under. That moment you hit the water hurts because you might as well bellyflop onto solid concrete. (of course, the water instantly moves around your body after you hit the water.). The more streamlined your dive is, the faster water will move out of the way, which means you won't get slapped by it instead.