She kept calm and swam on.

A California grandmother completed a jaw-dropping 29.6-mile, 17-hour swim through frigid, shark-infested waters off the coast of San Francisco, and while the apex predators kept their distance, she suffered jellyfish stings as some nosy sea lions and seals tagged along.

Amy Appelhans Gubser, 55, became the sixth person ever to make the daring swim through the treacherous stretch of sea from San Francisco Bay’s Golden Gate Bridge to the tiny Farallon Islands in the Pacific Ocean on May 11.

She is also the first to complete the grueling plunge from the mainland out — as opposed to starting from the islands and swimming to the shore — according to the Marathon Swimmers Federation [MSF], who named the Farallon Islands among its “Toughest 13” swims in the world due to the cold water, strong tides, unstable weather, and, of course, great white sharks.

“I know the sharks are there. I’m just grateful they didn’t find me interesting,” Gubser told The Post.

There was plenty of wildlife, though. She suffered numerous jellyfish stings during the swim, which “weren’t fun.”

She was also trailed by some curious seals and sea lions, who frequent the waters around the Farallon Islands with their predators.

As she approached the island, Gubser’s team said they spotted a couple of “half-eaten” seals, raising fears of a nearby predator. She’s thankful that they didn’t tell her about that.

She said she knew a shark attack was a real possibility. Her support team brought additional emergency medical equipment just in case and took on the heavy burden of potentially witnessing their friend get eaten alive.

“I don’t think people really understand that when [my team] signed up to be on this swim, it was possible that they wouldn’t see me come back,” she said.

Before dawn on Saturday, Gubser dove into the waters at the foot of the eastern span of the Golden Gate and started her historic trip — after about an hour delay so a massive container ship could pass.

The daunting endeavor was closely monitored by her seven-person support team, who followed her through the dangerous and capricious conditions on kayaks and boats. They fed her every half hour to keep her energy up and made sure she was all right as they documented her progress.One team member who followed in a kayak used a “shark shield,” which emits an electrical field that turns them away.

Over the course of her 17-hour, three-minute journey, Gubser faced varying conditions of fog, wind and frigid water temperatures — which, at times, reached a bone-chilling 43 degrees. She made part of her open-water odyssey through the ominous dark of night, which limited visibility to just her fingertips.

The daunting endeavor was closely monitored by her seven-person support team, who followed her through the dangerous and capricious conditions on kayaks and boats. They fed her every half hour to keep her energy up and made sure she was all right as they documented her progress.One team member who followed in a kayak used a “shark shield,” which emits an electrical field that turns them away.

“When [my team] signed up to be on this swim, it was possible that they wouldn’t see me come back.”

Mentally, too, she struggled to keep a level head as the hours rolled by like the waves she fought at times, but she persevered.

“It’s a challenge because you have to keep yourself entertained, but not overly thoughtful, because then you can get some negative thoughts,” she said, noting that singing songs in her head was useful.

“If you asked me what time it was during that whole swim, I couldn’t have told you. I just went into a place where I didn’t want to be aware of time passing,” Gubser added.

But her biggest enemy, it turned out, was the cold, which she had prepared for.

“I knew it would drop down into the upper 40s — but 43 was an unprecedented one for me. I’m not an ice swimmer,” she said. “It was really chill, and I knew when I would feel those icy patches that if I had to sustain those for a long period of time I wasn’t going to finish.”

Gubser, a fetal cardiology nurse at the UCSF Benioff Children’s Hospitals, had been planning the epic swim for five years but repeatedly had to put it off due to weather and tidal conditions.