RedactedNarrative: Boat operator reported he had just fueled his vessel and it appeared as if the fuel vent had not been working properly because gasoline kept spilling out.  Once fueled the operator turned on the exhaust blower and attempted to start the vessel.  An explosion from the engine compartment blew off the engine compartment cover striking a passenger knocking him unconscious with a laceration to the head.  The fire was quickly contained with a fire extinguisher.