RedactedNarrative: 7/18/16 Preliminary information is that a male passenger jumped into water to free line from port engine propeller. He didn't know how to do it and he was advised by owner to unwrap it in the opposite direction of the propeller. He surfaced asked a question, then went under water again and did not re-surface. According to persons on board it was a very short time later they saw a vague image of victim drifting from starboard to port below the swim platform. His body was recovered about 4 hours later. The starboard engine was running the whole time. Owner said it idles at about 800 RPM's and there is an air pocket that you can put your head in under the swim platform.  Still pending results of autopsy/toxicology results.
  
  ME result determined that carbon monoxide was a factor.
  
  Cause of accident boat design - swim platform is also used to muffle the boats twin power plants exhaust at water level to allow the water to absorb the gasses. This exhaust system has a history for producing high concentrations of carbon monoxide. Starboard engine was idling during the event.