RedactedNarrative: Operator docked his vessel and purchased 55.5 gallons of gas, evenly distributing the gas between 2 tanks. Cabin door was believed to be open with victim 5 & 6 inside. Operator/victim #1 starts engine. Victim #4 was on the dock washing the boat down. Victim #2 was seated in the left area of the vessel. Victim #3 and #6 were in the rear of the vessel. The twin inboard engines and bilge were located below the 3 individuals located in the rear of the vessel. At least one engine running. When operator started the 2nd engine there was an explosion from the bilge area knocking victims 3 & 6 overboard into the water. Victim #2 fell into the engine compartment which blew the floor off. Victim #4 was blown back while on the dock from the force of the explosion. Operator/victim #1, victim 7 and victim 5 were able to escape the boat and get on the dock. The dock attendant was able to access a fire extinguisher and put the fire out. It is unknown if the blowers were turned on and proper fueling procedure was followed or if there was some sort of malfunction.