Troopers responded to reports of a boat explosion and flash fire at the Millstone Marina Gas dock just after 2:15 p.m., according to the Missouri State Highway Patrol.
The incident report said the explosion happened when the driver started the boat after filling it with fuel. Investigators found the boat did not have time to properly ventilate. The boat also suffered a separate mechanical failure.
Officials said one man was ejected from the boat when it exploded. He landed in the water where he was later rescued.
Carl Harris, 42, was taken by life flight to University Hospital in serious condition, officials said.
The boat's driver 71-year-old Robert Baber, 6-year-old Kathryn Harris, 39-year-old Patrick Baber, and 48-year-old Cynthia Harris were all taken by ambulance to Lake Regional Hospital in moderate condition, officials said.
Kathryn Harris was the only person on the boat wearing a safety device, according to the incident report.
All five victims are from Kansas City, officials said.