Lt. Dan Mayer, the spokesperson for the Sheriff's Office, said the 27-foot boat from the '80s caught fire after the explosion and drifted to the shore where firefighters put out the blaze.
An explosion rocked the boat after the boaters filled up the gas tank but couldn't get the thing to start in the marina. So the marina staff towed the boat away from the docks, Mayer said, and when the boaters tried restarting it - the boat exploded.
Gas fumes had built up in the engine compartment, Mayer said. There were three people near the engine cover and the explosion blew them all 15 feet into the air and then into the water.
Amazingly, Mayer said, they were all OK - one person had just a minor injury.
The marina was not damaged by the explosion and law enforcement is looking at the potential cause.
U.S. Highway 34 was closed at mile marker eight due to the fire, the Grand County Sheriff's Office said.