The blast occurred around 2:15 p.m. during the refueling of a 1993 Chris-Craft vessel at the Millstone Marina Gas Dock, according to Missouri State Highway Patrol incident report.
Investigators attributed the explosion to a lack of ventilation and mechanical failure.
The highway patrol reminded boaters in a tweet to allow at least four minutes "after fueling & before starting the engine to remove gas vapors in the bilge."
The five people injured were from Kansas City, Mo., authorities said.
Patrick Baber, 39, was ejected from the boat into the water, where he was later rescued. Carl Harris, 42, suffered serious injuries and was airlifted to a hospital.
An ambulance took boat owner and driver Robert Baber, 71, Patrick Baber, the girl, and a woman identified as Cynthia Sterling, 48, to a hospital.
They suffered moderate injuries. The girl, Kathryn Harris, was the only one wearing a safety device, officials said.