Heart: The Marilyn Bell Story 2001 on netflix

This made-for-TV bio-pic is about Marilyn Bell, a Canadian teenager who, in 1954, was the first person to swim across Lake Ontario. She won the Toronto Canadian National Exhibition prize after Florence Chadwick, a then-famous American swimmer who was widely expected to win, dropped out in the middle of the race. Half of this heart-warming movie is devoted to the 21 hour swim in which the 16-year old Bell is exhorted by her pushy coach Gus Ryder not to give up.