The teen remains in critical condition after being pulled unconscious from the water.
It's not known why she suddenly slipped under the water while swimming with friends in Lake Ontario.
However, although it was the first official day of summer and temperatures throughout the month of June have averaged a seasonal 24°C, a Sea Grant Michigan Coastwatch map shows the surface temperature of the water was only about 5°C at the time.
Julie Evans, a swimming and water safety program representative at the Canadian Red Cross, said in water that cold, hypothermia can quickly set in.
“Water is generally considered cold at 21°C, but if you're in the water a long time, even in water as warm as 27°C, it can put you in danger,” she said. “(Yesterday's) water was very cold, and it's dangerous to go swimming in that kind of situation.”
As hypothermia sets in, the body concentrates more energy on warming the vital organs. The resulting loss of muscle control can quickly turn a strong swimmer into a weak, tired swimmer.
How can you tell if the water is too cold for swimming?
“If the water feels cold, it is cold, and any kind of cold water immersion can lead to hypothermia,” Evans said.
Before doing any swimming, it's a good idea to take some lessons, Evans added, and even if you're confident in your abilities, never go in the water alone.
“Always swim with a buddy,” she said, “and if you're not a strong swimmer, use a personal flotation device.”
If you get into trouble on the water, yell for help, float on your back and try to stay calm.
For more information on swimming and water safety, visit the Canadian Red Cross.