The boy was part of a group of children from Sunchine Summer Camp, a day camp that was using the beach.
He was found in the water at 10:45 a.m. ET by one of the supervisors of the group.
The child was removed by lifeguards, who performed CPR, before paramedics took over.
Urgences Santé spokesperson Bob Lamle said paramedics managed to resuscitate the boy in the ambulance en route to hospital, where he was met by his family.
The boy is in critical but stable condition in the pediatric intensive care unit of the Montreal Children's Hospital.
Lamle said he has a strong pulse and good blood pressure. He said the next step is to take a brain scan to determine if there is any damage.
Some witnesses at the scene were treated for shock, but were not transported to hospital, police said.
The beach is closed as investigators speak with witnesses.
Nathalie Lessard, spokeswoman for the park, said the supervision ratios at the time of the incident were respected. There were 64 children in the group and 21 supervisors. That's in addition to the lifeguards stationed at the beach.
She said in the 20 years the beach has been operating, this is the first such incident.
Swimmers are being redirected to the Aquatic Complex in the park.