Marilyn started her swim completing against a swimmer from the United States, Florence Chadwick. The Canadian National Exhibition offered Chadwick 10,000$ to make the event public. Marilyn was also offered the money as well, but she did not accept it. Into the race, Florence Chadwick was forced to give up due to stomach pains and vomitting. Marilyn continued on and completed the swim in 20 hours and 15 minutes. Making her the first person to swim across Lake Ontario.