RedactedNarrative: Operator launched boat from marina with 8 month pregnant finance onboard.  Passenger was seated just to the port side of the engine cover.  When operator started the engine an explosion occurred throwing the engine cover off toward the bow of the vessel.  Flames from the explosion burnt the passenger on the face, neck, chest, and arms and was treated at a nearby hospital.  An onboard fire extinguisher was able to put out the fire.  The operator did not properly ventilate the engine compartment following fueling.  The engine was equipped with a back fire flame arrestor but it was not properly attached.