Ontario Provincial Police said they were called to the site of a possible drowning in front of a Tiny Township cottage just after 10:30 a.m. Saturday.
Police said the 17-year-old swimmer ran into difficulties while in the water with his friends.
He was pulled from the water with no vital signs, and nearby cottagers performed CPR.
The young man was pronounced dead at the scene. OPP have identified him as Trevaughn Miller, 17, of Ajax.
Police said an autopsy will confirm the cause of death.