A boy was seriously injured when he lost control of his water bike which left a lake, went across a car park, through a hedge and landed on a car.
The accident happened on Wednesday evening at the West Country Water Park at Winterbourne near Bristol.
Police said the car driver was shocked but unhurt. The eight-year-old boy is being treated at Frenchay Hospital.
The park was shut while investigations were carried out and the Health and Safety Executive has been informed.
A spokeswoman for Great Western Ambulance Service said: "We sent an ambulance and rapid response vehicle to the incident.
"The boy had serious head injuries. He was in a critical condition and was unconscious."
The Royal Yachting Association (RYA) governs water bike activity.
Spokesman Paul Mara, said: "Currently there is no legislation on the use of personal watercraft.
"However, the RYA does provide and promote its training scheme, which covers all aspects of the safe operation of personal watercraft."