RedactedNarrative: Owner of vessel had recently had his vessel worked on and mechanic told him that the it was running a little rough.  He suggested that he run the boat and take on some fresh fuel.  Operator and his teen children took the vessel to Cape Ann Marina to get fuel.  He took on approximately 50 gallons of gas.  There was an explosion on board in the area of the engine compartment.  Blower fan was operational and used prior to the explosion and he indicated that he smelled no fuel or other vapor.  His son was sitting on the engine cover seat cushion and he thought his daughter was sitting in the passenger seat at the time of operation.  His son and the cushions had blown off the compartment which gave the operator immediately access to engine compartment with fire extinguisher.  He was unable to put down the flames so he and his children jumped into the water.  A small passing boat pulled along side and towed them to the marina where they were removed from the water.  The filling of the fuel tank, minutes before explosion most likely allowed fuel to leak from a hole, loose clamp or fitting in or on a fuel supply line, tank or vent.  The leak allowed fuel to enter the bilge.  The gasoline while in the bilge began to evaporate while mixing with the oxygen in the engine compartment eventually reaching the explosion limit.  
  
  Investigating officer concluded that the explosion was likely caused by a spark from an unknown source (ie non marine engine parts) or fuel contact with the exhaust manifold in the bilge area while dangerously high gasoline vapor were still present.