RedactedNarrative: Operator/ Victim 1 was preparing to launch two PWC off of a trailer. He was standing in the water next to a Polaris Jet Ski, and when he attempted to start the engine the PWC burst into flames and blew the sear off of it. During this explosion, fuel also was thrown from the machine and ignited on top of the water. There was a three year old male, Victim 2, standing on the second PWC on the trailer. He received burns on his lower legs. Medical and fire were dispatched to the scene. Upon my arrival all flames had been extinguished, both on the water and the PWC. I began to gather information about what may have caused the fire. I asked Operator/ Victim 1 if he had done any recent mechanic work to the machine and possibly replaced any parts with automotive parts instead of marine. He indicated that the starter on the other PWC had been replaced, but that it had been done at a shop. I began looking through the engine compartment and could not find any melted wires. The spark plug wires were intact and in good condition. The spark plug boots were also in good condition and seated snugly on the spark plugs. When I looked in the front compartment of the PWC I could see two different pools of liquid. One was smaller than the other and located at the very front of the compartment. I stuck my finger into the fluid, and then was able to smell the odor of gasoline. I asked if Operator/ Victim 1 had recently fueled the PWC. He indicated that he had. I asked how he had done it, and he said he had taken it to a gas station. I asked if he had possibly overfilled the tank, and he told me he had stopped at the automatic stop on the fuel pump. I showed him the fuel in the bottom of the machine and told him in order to have that much fuel, he may have a leak in the fuel system somewhere. Operator/ Victim 1 told me that they had recently had both PWC out on the lake and that there had been no problems. I gathered registration and insurance paperwork for the PWC. While I was doing that Medical and Fire arrived. We had them check on both Operator/ Victim 1 and Victim 2. It was advised that they self transport to the hospital. They wanted Operator/ Victim 1 to have his lungs checked, and wanted to treat the burns on the leg of Victim 2 better.