She fell from the Splash Canyon ride at Drayton Manor in Tamworth.
The girl, thought to be from the Leicester area, was flown to Birmingham Children’s Hospital but died shortly after arriving.
West Midlands Ambulance Service said it had been called at 2.18pm, with an emergency response team on the scene within seven minutes.
“Crews discovered a girl with serious injuries who had been rescued from the water by park staff,” said a spokesman.
He said ambulance staff had given the girl “advanced life support” and that a doctor continued treatment in the helicopter.
Feeona Fletcher, who was on a nearby ride, told Sky News: “We saw the park golf buggy thing coming down and the guy was shouting ‘clear the way, clear the way’.
“Suddenly two paramedic cars came flying down through the park and into that area.
“By the time we got off the carousel, maybe a minute or two, there were fire engines, ambulances, police cars.
“The air ambulance was over our heads. It was so fast.”
Ms Fletcher said she had previously been on Splash Canyon.
“You go around floating on water, being bounced off boulders and stuff… You can bounce off other boats as well, so it can be a sharp knock,” she said.
Drayton Manor’s director, George Bryan, said staff were “immediately on the scene” after the girl fell in.
Appearing visibly shaken, he told reporters: “We’re all truly shocked and devastated and our thoughts are with her family and friends at this tremendously difficult time.”
Specially trained police are supporting the girl’s family and the Health and Safety Executive said it was making “initial inquiries”.
The ride was shut after the incident, but people already inside the park were allowed to stay until the usual closing time of 5pm.
Drayton Manor will close on Wednesday as a mark of respect.
Splash Canyon opened in 1993 and is described as Drayton Manor’s “very own river rapids”.
It has a maximum capacity of 126 people – 21 boats with six people in each – and a minimum height of 0.9m.
The boats have a central circular bar to hold on to, but in footage posted online the seats do not appear to have any seat belts or other restraints.
The park’s website says: “Swerve around corners, splash through the waves, and get soaked as your boat is swept along our fast-flowing rapids.
“No two experiences are ever the same, so grab your friends and family, climb on board one of our circular boats, and get ready for a wild ride through the water!”
Drayton Manor – which includes a Thomas the Tank Engine-themed area – opened in 1949 and is the UK’s fifth most popular theme park.
An incident on Alton Towers’ Smiler ride in June 2015 led to two teenagers losing a leg and left 14 others injured.