RedactedNarrative: Unit 1 was traveling south on Lake Marion when a passenger that was seated in the front of the boat lost his hat.  Passenger stood up in an attempt to catch his hat, meanwhile the operator let off of the throttle.  This action caused the passenger to lose his balance and fall overboard and under the vessel.  Passenger was struck in the left arm by the propeller as the vessel continued forward movement.  Passenger was recovered and pulled back aboard Unit 1.  Unit 1 returned to Packs Landing where passenger was transported via helo to hospital for treatment.