RedactedNarrative: Eight persons were aboard Vessel 1 sightseeing for approximately one hour and were returning to dock when the incident occurred.  Vessel 1 slowed to drifting approximately 70 yards off the beach on the east side of the entrance to the marina.  The Victim decided to swim to the beach from that location.  The Victim voluntarily exited the boat off the starboard side of the swim platform and began swimming.  The Victim got to approximately 60 feet from shore when an unidentified personal watercraft passed between the Victim and the shore, on plane, then turned 180 degrees and passed between the Victim and the shore once again.  At this time the Occupants on Vessel 1 noticed that the Victim was in trouble.  The Victim raised his hands for help, then disappeared underwater and did not resurface.  The Operator and two of the Occupants aboard Vessel 1 jumped into the water and tried to reach/assist the Victim, but were unsuccessful in their efforts.