RCMP says the girl suffered serious injuries to her leg and torso in the accident on the Newell reservoir near Brooks on Friday night.
Police say her injuries could have been more severe if not for the quick actions of the boat's operator, an 18-year-old woman, who came to the teen's aid.
The girl was taken by air ambulance to hospital in Calgary.
RCMP say neither of the boat’s occupants was wearing a life jacket.
Alcohol has been ruled out as a possible factor.