“At first, I didn’t massively enjoy it,” Danny Cipriani, a bearded British rugby fullback who was spending his summer in Malibu, said the other morning, over espresso mixed with the oils of coconut and red palm*. He meant the ice bath that his hosts—the big-wave surfer Laird Hamilton and the retired volleyball star Gabrielle Reece—prescribe as part of a radical underwater weight-training routine that they lead for friends at their house three times a week. The secret to ice, Cipriani said, is to endure past hurt to numb (two minutes for him) before fully submerging for twenty seconds. “Then you have that euphoric rise, and you’re happy like a thunderbird.”

Outside, a dozen figures, muscled and inked, were arrayed as if on an amphora at the deep end of a pool. There were alumni of the N.F.L. and the N.H.L., a two-time Olympic gold medalist, a vegan superfoods hunter, a couple of M.D.s, and a jujitsu black belt who keeps the riffraff from Oahu’s North Shore. Reece, who is six feet three, with long blond hair, watched her husband applying himself to a line of dumbbells. Every few seconds, weight in hand, Hamilton popped up to the surface, caught a breath, and descended, trailing bubbles. “It’s harder than it looks,” Reece said.

Hamilton, who is fifty-two and bearish, with a freckly tan, grew up on Kauai in a house with no indoor bathroom. At the local pool, he liked to squat down in the shallow end and then burst up through the water: the active boy’s version of an underwater tea party. To expand his lung capacity, he’d grab a rock and run along the ocean floor, holding his breath for as long as he could. Then he started exercising underwater, wearing a weight vest. One night a decade ago, he had a dream about jumping up and down in the water, breathing rhythmically, as he’d done in childhood. In the morning, he and Reece began to develop the routine, which they call XPT™—extreme performance training—and which, after years of testing on friends, they have begun to promote through retreats in Malibu and on Kauai, where they live during the big-wave season. Videos, apps, and books are in the works.

“O.K., how about some Reeces?” Hamilton asked, pulling himself onto the diving board. Reece Hamilton, a strapping twelve-year-old, swam over to her dad. Because, at four, she liked to retrieve weights one-handed from the bottom of the pool, a one-handed squat press is a Reece, while a Gabby is an underwater stretch with a single jump. The Rubin—named for the music producer Rick Rubin, who is a regular at the Hamilton-Reece pool—is a submerged jumping jack done with a weight in each hand. Hamilton said that, if anyone enters the pool overconfident, it doesn’t last long: “I once asked Wayne Gretzky why hockey players are so humble. He said, ‘We get rid of the cocky guys in the minor leagues.’ There’s always a more gnarly guy. Water is the great neutralizer.”

Enough talk. Gabrielle Reece snapped on a swim cap and held out a mask to an appropriately trepidatious visitor. “Take two twenty-pound weights and do some jumps,” she said, indicating a section of the pool floor that sloped downward. Her student complied, lugging the weights toward an underwater staircase, up which a man was sprinting with a huge dumbbell in each hand. The student, jumping, had the sensation of being a deranged pogo-stick rider, likely to drown. Piano music flowed through underwater speakers. A mermaid appeared out of the blue-green: Gabrielle Reece, with pointers. (“Try to go straight up and down.”)

In the sauna, which was heated to two hundred and twenty degrees, Randall Wallace (“Braveheart,” “Pearl Harbor”) chatted with Frankie Harrer, an eighteen-year-old professional surfer, about the state of her soul. (Solid.) Neil Strauss, the former rock critic and author of “The Game,” came in. He calls the sauna the “truth barrel”; he and Reece have used it as the location for a podcast about “life optimization.” More friends stopped by. John McEnroe, who had been cycling up the canyon, peeled off his shirt and made a beeline for the ice tub, where he lay palely for several minutes before bolting upright, re-dressing, and calling over his shoulder to Hamilton, “I want to talk to you about the breathing. I have a match next week.”

When Cipriani finished his workout, he headed down the hill for lunch. Even with the thousand-calorie coffees, he said, he had lost ten pounds in two weeks and healed a bum shoulder. He felt confident that he’d return to England with a competitive advantage. “Ninety-eight per cent of the players are in Marbella, Ibiza, Las Vegas, and they’ve been drinking for two to three weeks straight,” he said. Ruggers typically age out around thirty, Cipriani noted, and he is twenty-eight. He used to be scared of retirement, but now he has a plan, which involves swimming, breathing, espresso, and ice. “I’m just going to come and hang out here and become a Hamilton,” he said.