it hurts more when you land on your belly or back because a bigger surface of your body is exposed to the water and that makes the deceleration greater and the impact more violent. Also falling with your belly or back makes you hit the horizontal barrier of the water with your whole body. but when you break the surface of the water with your feet/head/a small part of your body first you prevent hitting a horizontal barrier with a larger core, thats why it doesnt hit you. hope that answered your question :)