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Sumaya Haddin of Miami tracked Ms. Nyad’s swim before her family’s trip to Key West this weekend. She was surprised to see Ms. Nyad’s flotilla from a parasail off Smather’s Beach on Monday morning. She thought Ms. Nyad wasn’t due for another day.

“You couldn’t see her; you could just see the boats. It was very exciting,” she said.

Ms. Haddin said Ms. Nyad still had her fighting spirit when she got to the beach.

“Getting into the ambulance, she had her peace sign up, her fist up. She was still fired up,” Ms. Haddin said.

Australian Susie Maroney successfully swam the Florida Strait in 1997 with a shark cage, which besides protection from the predators, has a drafting effect that pulls a swimmer along.

In 2012, Australian Penny Palfrey swam 79 miles toward Florida without a cage before strong currents forced her to abandon the attempt. This June, her countrywoman Chloe McCardel made it 11 hours and 14 miles before jellyfish stings ended her bid.

In 1978, Walter Poenisch, an Ohio baker, claimed to have made the swim using flippers and a snorkel. Critics say there was insufficient independent documentation to verify his claim.

Ms. Nyad first came to national attention in 1975 when she swam the 28 miles around the island of Manhattan in just under eight hours. In 1979 she swam the 102 miles from North Bimini, Bahamas, to Juno Beach, Fla., in 27.5 hours.

Ms. Nyad is also an author of three books and a motivational speaker and has been a reporter and commentator for National Public Radio.

• Associated Press writer Anne-Marie Garcia in Havana contributed to this article.