How do stand up jet skis drain?

They most commonly use a suction bailer system. This is a very simple and very effective bilge draining system. Essentially, a hose is inserted into the thrust portion of the pump in such a way that a vacuum is formed using a venturi effect. The other end of the hose goes to a box with holes in it at the bottom of the jet ski. Water in the jet ski is sucked into the bailer box and then into the pump and out the back.