The teenager scaled the outdoor pool's fence after it had closed for the evening, police spokesperson Const. Anthony Cantelmi said.
Police are looking into the possibility that the boy was playing soccer with friends in an adjoining field when the ball went over the fence into the pool area, and that the teen went to recover it.
Cantelmi said some of the boy's friends began to yell after noticing him in distress underwater.
"Now it's in a residential area, so the neighbours that heard the screams called 911 and the officers that arrived on-site noticed that as they were getting there, the group was finally getting their friend out of the pool," he said.
Police and paramedics tried rescuscitating the boy, but he was pronounced dead after being taken to hospital, Cantelmi said.
The pool is in Raymond Park, behind LaSalle Community Comprehensive High School and across the street from a row of semi-detached homes along Bishop Power Boulevard.
The fencing around the pool area is one storey high and topped by barbed wire. A segment of the barbed wire was damaged Wednesday morning, though it has since been repaired by a city worker.
Police said they won't lay any trespassing or other charges in the incident.