The accident happened just before 3:45 p.m. Friday at Paul XI pool in the community on Montreal's south shore.
Two other people were hurt — a 6-year-old boy suffered severe injuries and a 40-year-old woman suffered minor ones.
Const. Jean-Pierre Voutsinos of the Longueuil police said the young woman, who was a municipal employee, was declared dead at the hospital.
An intense thunderstorm was preceded by high winds that led to downed trees all over the greater Montreal region.
"At a quarter to 4 p.m., the winds were already quite strong," Voutsinos said.
The Longueuil police and the CSST, Quebec's workers' health and security agency, are investigating the death.