RedactedNarrative: Vessel 1 was stopped in the open water of Lake Powell, AZ, up Navajo Canyon.  The vessel was being used as a swimming platform.  OPERATOR departed the vessel voluntarily as a swimmer.  OPERATOR experienced panic upon swimming in such a large body of water with the accompanying waves.  Wind and waves caused Vessel 1 to drift away from OPERATOR faster than he could keep up.  Vessel 1 was operated back to OPERATOR, bow-on.  OPERATOR swam to the stern of the vessel and accessed the swim-ladder to climb aboard the rear-deck.  As OPERATOR was climbing the ladder, his left knee hit the unmoving propeller of the starboard-side outboard motor.  That contact resulted in a 1" or 1.5" laceration on the top of his knee.  He then finished coming aboard and received first-aid for his lacerated knee and was later transported to the Page, AZ hospital where he received three stitches on his knee.