In fact, the 64-year-old swimmer needed a customized face mask to help protect against the deadly box jellyfish, the venomous creatures who cut short her previous, fourth attempt at the grueling journey.
The customized silicon mask may have looked like something out of a horror movie, but it was critical for Nyad to complete the 52-hour journey.
"I was grateful for it," she said. "I knew I wasn't going to be stung at all. I felt 100 percent prepared for the jellyfish."
Nyad said the mask wasn't perfect: It's narrow mouth opening caused her to swallow "tremendous volumes" of water, causing vomiting, dehydration and discouragement in the 13 hours Saturday night and Sunday morning she needed the mask.
"That night was hell on Earth, it really was," she said.
Fortunately, the jellies dispersed by Sunday afternoon, and she was able to swim the rest of the way without the mask.
Nyad's fourth failed attempt to swim between Cuba and Key West was halted when the box jellyfish found the only exposed part of her body: her lips. She had worn head-to-toe protective gear otherwise, but the jellies managed to find bare skin, causing, as Nyad described it, "the paralysis, the otherworldly sensation of being burned alive."
The mask was designed and produced by Stefan Knauss, a prosthetics expert, who had Nyad try out the design by swimming through a swarm of hundreds of box jellies in June.
"As difficult as the swimming was, I was not stung once," she said. "Those deadly tentacles could not penetrate."