RedactedNarrative: Operator purchased over 100 gallons of fuel from the marina.  All passengers boarded and v1 was pushed away from the marina. Operator stated he turned on the vent blower and it ran less than 1 minute without the rear hatch open.  He attempted to start the port engine but it didn't turn over at all.  
  
  He then attempted to start the starboard engine and it didn't turn over either.  He again tried to start the port engine and heard a loud explosion when he turned the key. 
  
  Operator turned to see several members of his family burned and on fire jumping into the water. He then opened the rear engine hatch to check for fire.  He stated there was no fire and the onboard fire suppression system had not discharged. 
  
   Injured persons were transported by ambulance and air evac to hospitals.  Passengers 5 and 7 were below deck in the bow cabin when the explosion occurred. All other passengers were in the stern area over the engine hatch.
  