"I've learned that if you see a beautiful calm sea like this you don't say that is the way it is going to be forever ... I admit I'm scared, I know what's out there," Nyad, 64, said, as she prepared to enter the water at Havana's Hemingway Marina, just west of the capital.
Her biggest challenges during the 103 mile (166-km) swim, apart from fatigue, will be the poisonous jelly fish that float through the Florida Straits, the sharks, the man o'wars, storms, waves and the powerful and unpredictable Gulf Stream, the mighty ocean current that flows west to east between Cuba and Florida.
If she succeeds, Nyad, who has failed on four attempts to make the crossing, would become the world record holder for the longest unassisted open ocean swim, meaning no shark cage or body suit to protect her. She was not using flippers.
Nyad said on Friday that jelly fish stings were one reason for her undoing on an attempt last year, but this time she would be wearing a protective silicone mask to ward off their paralyzing stings.
She said the custom-made mask slows her and makes it more difficult to breathe, so it may take up to three days to complete the swim.
Nyad will have a small fleet of five support boats and a crew of 35 to keep her on course through the strong Gulf Stream current, provide food and water and keep away sharks.
She told reporters on Saturday this would be her last attempt.
"This is the last time, this is the end of the journey as they say, 100 percent," she said.
The treacherous body of water known as the Florida Straits is the holy grail for marathon swimmers. It has been conquered only once, by Australian Susie Maroney, who used a protective cage at age 22 during her 1997 swim. The cage glided on ocean currents and enabled her to make the journey in just 25 hours.
A native New Yorker, Nyad was raised in south Florida by a French mother and Greek-Egyptian stepfather.
She is an advocate of improved U.S.-Cuba relations and says she repeatedly sings the Cuban song "Guantanamera" as she strokes her way toward Florida.
"I want to be a little part in the relations between Cuba and the United States," she said on Friday.
Her long-distance accomplishments include swimming around Manhattan island and a swim from the Bahamas to Florida in 1979.
(Additional reporting by Nelson Acosta; Editing by Vicki Allen)
