Operators of the ride, called "Shoot to Thrill," immediately shut it down and rushed to help passengers out of the fallen boat, as did police officers and passersby.
Witnesses said that several passengers were caught underwater for several minutes.
"We jumped in and helped them get out," Matthew Orr, a park visitor, told the Register. "If we didn't help, I don't even know what would have happened.
"It took nine or 10 people to pull the car up," he added. "We had the car sitting sideways."
Six people were treated for injuries at the park while another person was taken to a local hospital.
Cedar Point said in a statement that park officials had notified state authorities and were "investigating the incident."
"The safety of our guests is our number one priority," the statement read.