SAN PEDRO, Calif. — The Bottom Scratcher stopped 300 yards off San Nicolas Island on Monday at 5:30 a.m., and Zach Jirkovsky was among those who stirred from his bunk. An environmental engineer, he had been to the island two years before for work, and ever since he had been thinking about a return trip and about swimming the 70-mile channel back to the Southern California mainland.

It was far too great a distance to bridge alone, so he asked his friends from a swim group called the Deep Enders in Ventura, California, to take on the challenge as a relay. Most of the Deep Enders were collegiate swimmers once upon a time, and all had serious channel crossings on their résumé.

They did not need much persuading.

Jirkovsky, 34, and Tamie Stewart, 42, stood on the deck of the 63-foot boat and marveled at the island before they jumped into the water and, escorted by a kayaker, made the easy, 10-minute swim toward dry land. One must receive permission to visit the island because it is a U.S. Navy base, and Jirkovsky had the OK.

Stewart stayed in water waist deep as Jirkovsky emerged from the sea at Cissy Cove, greeted by three sailors, all armed. Channel swimmers must begin “toes dry,” and Jirkovsky took several steps beyond the tide line to make sure their swim would qualify. Then he placed his goggles over his eyes, jogged back toward the sea and began to swim.

The sailors honked their horns and flashed their blue and red lights in salute. The captain of the Bottom Scratcher sounded her horn too, and the Deep Enders and their support team cheered. If all went according to plan, within 36 hours, they would make open-water swimming history.

Jirkovsky and Stewart swam back toward the boat, and only Stewart got out. She would be the last of the six-person relay team to swim a shift. Each would swim for an hour before yielding to the next, and for the crossing to officially qualify, the team would have to maintain that order and each athlete would have to spend an hour in the water when the time came. If one of them quit for any reason, the attempt would fail.

Jirkovsky kept swimming, following the kayak, which followed the support boat. The current was at his back. After an hour, the team’s leader and coach, Jim McConica, 64, jumped in, the first shift change. By the time John Chung, 45, tapped him out, the Deep Enders were off to a splendid start.

By 3 p.m. Monday, though, the team was fighting current and dealing with a 7-foot swell. The battle peaked 20 miles into the swim as they approached Santa Barbara Island that night.

“The island is shaped like an airplane wing,” McConica said, “and the volume of water sweeping around that island was hugely powerful and cut our speed.”

Up until then, the swimmers had been averaging 5,200 yards an hour. In the face of that current, McConica still managed 2,600 yards during his hour, but others could barely swim 900 yards.

“We were almost at a dead stop,” McConica said. “It was a difficult moment, and the moment lasted about four hours.”

It did not help that it was pitch black.

“All of us have swum in the middle of the night before, but it had never been quite this dark,” added Stacey Warmuth, 59, who drew the team even with Santa Barbara Island and around its southern point.

“There was no moon,” Stewart said, “and the water was moving in every direction. I felt like I was in a washing machine.”

With the worst of the swirling current behind them, the team still fought a negative current, wind chop and swell until well after sunrise.

With a record heat wave on blast, the air was warm through the night and the water temperature was over 70 degrees almost the entire way, so hypothermia was not an issue — but jellyfish were. Open-water swimmers are not permitted to wear wet suits or rash guards on crossings, and all of the swimmers were stung repeatedly. Stewart soon had welts up and down both forearms.

There were also two shark sightings. Late Monday afternoon, the team saw a 5-foot shark trailing Chung, though it kept its distance. On Tuesday morning, after clearing Catalina, a 6-foot shark passed in front of the escort kayak while McConica was swimming. The boat captain asked McConica to swim closer to the boat and warned his teammates that if the shark drew any nearer, he would have to pull McConica from the water and end their quest.

But the shark disappeared, the current turned at midmorning Tuesday, and for the final five hours of the swim the athletes enjoyed a plus current, their every stroke infused with the momentum of the ocean. Warmuth took the final shift, as the Deep Enders closed in on the Palos Verdes Peninsula.

“I was just gliding across the water,” Warmuth said. “It was very fluid, the water was warm and glassy. It was an amazing feeling.”