Two children said they had met a child in the water at Johnson’s Beach.
"As the two children went to shore they lost sight of the other child and became concerned that she was in trouble," Barrie police said in a news release.
A lifeguard was notified about the incident.
Police were called at about 3:45 p.m. They say they have yet to confirm that a child is missing, and they have not received any reports from parents of a missing child. They also say there was nothing left behind on the beach.
Three police boats are taking part in the search effort. At least one of the boats is equipped with sonar.
At 8 p.m., the search was suspended briefly when the marine units responded to a boating emergency, but it resumed about a half hour later.