RedactedNarrative: Operator had beached his jet boat on a sandbar to allow children to get out and swim. When done swimming, all occupants got back in boat and operator started the engine. Operator put transmission in drive but boat did not move and operator noticed it sounded odd. He increased the throttle and a failure in the jet-drive system began pumping water into the boat, flooding the hull immediately and sinking the boat. Upon retrieving the boat the next day and examining the engine/jet drive, it was determined that the jet nozzle had become packed with mud/sand and the impeller inspection plate had become dislodged due to the resulting pressure build-up, causing the jet drive to pump water into the boat instead of out the jet nozzle.