The car plunged into the river and the driver was unconscious when he was pulled from the water, Madison Police Chief Bob Wolf said during a news conference. Two people were in the car.
Four of the injured, including the driver, were airlifted to hospitals in Louisville, Ky., about 40 miles away, Wolf said. One was sent to Kosair Children's Hospital and the others were taken to University Hospital, officials said. Their conditions were not available Sunday evening.
Seven people were treated and released at King's Daughter's Hospital in Madison, hospital spokeswoman Nadja Boone said.
David Edds, of Owensboro, Ky., was at the Madison Regatta and told The Associated Press he heard a loud noise and saw a car in the air.
"I saw one young lady just frozen in her path, just standing at the water's edge and the car ran directly over the top of her," he said.
Spectators rushed to help the injured, Edds said.
"There was just a trail of folks laying on the ground in the path of where the car had gone," he said.
The Madison Regatta's main event, the Indiana Governor's Cup, was canceled after the crash. The Associated Press left messages seeking comment with race officials.
Madison is in southeast Indiana, midway between Louisville, Ky., and Cincinnati. The annual race for unlimited hydroplanes draws thousands of people to the riverside community of about 13,000 every Fourth of July weekend.