She's been trying to become the first person to make the swim without a shark cage.
Nyad turns 63 on Wednesday. She first tried to cross the Straits of Florida in 1978.
Update at 9:30 a.m. ET. Nyad's Team Says Storm Made It Impossible To Go On:
In a new post on Nyad's website, her team reports she was "fully alert and articulate" when she got into the escort boat and wanted to get back in the water to finish the swim. But "with lightning, thunder, and roiling winds tossing her tiny escort vessel up and down on the waves like a paper cup" it was decided that "for the sake of the safety of her team and herself" they needed to motor on to Key West.
"It took Mother Nature's biggest force — the weather — to get her out of the water," the post says.
And it adds that Nyad "was stung by box jellyfish nine times on Monday night alone, with sharks surrounding her as a team of divers labored for hours in the darkness to discourage them away."