“Day 1 and Day 2, we want flat calm,” said Mr. Marchand. “By Day 3, well, it’s hard to get three days of flat calm. But even in a flat sea, you can get a squall — rain, thunder for 15 minutes to an hour, and then calm again. You can’t dodge them. And it’s almost an impossible swim as it is. You have to keep in a straight line.”

Two men in kayaks will follow Ms. Nyad’s every stroke. They will hold a shark shield — neoprene rods that emit electrical waves to zap sharks that come too close. The waters between Cuba and Key West are a notorious shark playground. But the shield is not foolproof. Just in case it fails, as it did last year in the Caribbean when another woman was on a marathon swim, four shark divers with spears will be onboard, ready to jump.

Some contraptions are decidedly low-tech. As Ms. Nyad swam recently off the shore of Key West at a pace of two miles an hour for nine hours, she followed a white streamer beneath her, like a line in a pool. Keeping her eye on the boat had been difficult, and she would often veer off course. Crew members found that if they attached a streamer to a long pole and dropped it into the ocean, she could see it underwater. Problem solved.

Another advantage Ms. Nyad has, three decades after her first attempt to cross the straits, is the emphasis on sports nutrition. In 1978, Gatorade stood mostly alone in the field, and even then it was a niche drink. Now Ms. Nyad has her predigested proteins to drink and gel blocks of electrolytes to suck on to keep her calorie intake high and her body hydrated and balanced.

Still, her doctor worries about hydration, starvation and keeping her body warm in the water. The ocean must be at least 86 degrees, which sounds warm unless a person is in it for 60 hours.

“The big concerns are, can she stay awake, focused and hydrated,” said her physician for the swim, Dr. Michael S. Broder, an associate clinical professor at the University of California, Los Angeles, School of Medicine. Some days her long swims have made her sick and other days they have not. “Because nobody else has ever done this, it’s not clear what causes all the things she has experienced,” Dr. Broder said.

The mental game, too, could doom her. Conditions in the ocean are nothing if not hostile. There will be no gorgeous sunset she can watch to buck up her spirits, as there would be if she were running or cycling. No music playing or conversation to distract her from her pain.