RedactedNarrative: VESSEL 1 WAS DRIVING BEHIND VESSEL 2 ON CHATFIELD RESERVOIR APPROX. 500 YARDS SE OF THE TOWER.  VESSEL 2 SLOWED AND VESSEL 1 STRUCK THE BACK OF VESSEL 2, CAUSING BOTH OPERATORS TO FALL OFF THEIR PWC'S INTO THE WATER.  OPERATOR 2 SUSTAINED A LACERATION TO THE BACK RIGHT SIDE OF HIS HEAD AND A CONTUSION TO HIS ELBOW.  OPERATOR 1 WAS ISSUED A CITATION FOR UNLAWFUL OPERATION OF A VESSEL IN A CARELESS MANNER.