RedactedNarrative: Owner/operator flipped on the blower switch and tried to start the engine but it didn't turn over with battery 1 so he switched to battery 2 and the engine started. He then switched to both in hopes of recharging battery 1. He pulled into the front gas dock, tied up and turned everything off. He opened both the bottom and top of the i/o compartment, purchased $100 worth of gas. He used the key to close the cap on the tank and the I/O compartment was still open. He turned on battery 2 and then turned the key and he there was a flash, he felt and heard a "whoosh" and then the stern was engulfed in flames. He was next to the captains chair and his son was near the port stern. He pushed them into the water and jumped in after them but midway he realized his twins were in the cabin. He grabbed the cleat as he was going over and did a 180 and grabbed the ladder and pulled himself back up. He grabbed the twins and threw them into the water and jumped in after them. He told everyone to swim as far away as they could. One of the local lobster men was there in his skiff and helped them get to the ramp. Operator's routine was to start the blower and turn the engine on right away. He would not wait the recommended time for fumes/vapor to vent or check engine compartment for gas/fumes before he started engine. He stated in this accident he did the exact same as he always did to start the engine. Failure to vent is the cause of this accident.