RCMP Const. Lesley Smith says police received a report about two young males yelling for help on an island about 30 metres from shore around 5 p.m. on Sunday.
The 16- and 17-year-old boys told police the trio swam across the lake toward the island but halfway across, their friend panicked and went under the water.
Smith says police and conservation officers went out in their boats and that a search and rescue crew and an ambulance were also deployed.
The boy's body was recovered soon afterwards.
His friends were treated for shock and non-life threatening injuries, and the coroner's service is investigating the tragic incident.