RedactedNarrative: Owner put his vessel in the water for the season at the boat ramp.  He attempted to start the engine at which time he noticed smoke and flames coming from the engine compartment.  He attempted to put out the flames with a fire extinguisher and sustained injury.  He was unable to put the flames out and exited the boat.  Vessel burned until it sank which extinguished the flames.  Preliminary cause is that the ignition of fuel vapor is the cause of fire and resulted in a total loss of vessel.