On the California coast on the morning of July 4, 1952 a fog was rolling in. Twenty-one miles to the west, on Catalina Island, a thirty-four-year-old woman waded into the water and began swimming toward California, determined to be the first woman to ever swim the twenty-one-mile strait. Her name was Florence Chadwick, and she had already been the first woman to swim the English Channel in both directions.

The water was numbing cold that morning, and the fog was now so thick that Chadwick could hardly see the boats in her own party—there to scare away the sharks. As the hours ticked off, she sw…