Scarborough resident Sue Williams was enjoying a picnic at a Lake Simcoe beach on Canada Day when she noticed her friend running toward her for help.
"(She was) saying she thinks that a girl has drowned. She was asking for help to call 911 and for someone to do CPR," Williams said. "I walked over there and saw a crowd of people. A man was holding a girl's head."
A woman began chest compressions while Williams, 26, blew air into the victim's body.
"Then water started coming out of her nose and mouth," said Williams. "She was still unconscious so we kept doing CPR. We were working as a team."
The victim turned out not to be a girl, but a 42-year-old woman, also from Scarborough. York Regional Police did not release the victim's name.
The victim, who was revived at the scene, was airlifted to a hospital in Barrie where she died two days later.
York Regional Police spokesperson Const. Gary Phillips said the victim didn't know how to swim and somehow got separated from her family and friends.
Williams said she was trained in CPR about six months ago and it was still fresh in her mind.
"I was recently reviewing it in preparation for my summer camp job," she said. Williams is working this summer at the Infinity Leadership Day Camp in Scarborough.