To see Clay Marzo drop into an enormous wave, carving under its barrelling lip then emerging, in a vibrant flash, to throw his surfboard into a series of elaborate leaps and twists, is to witness a jaw-dropping display of raw sporting talent.
Watch the professional surfer totter on to dry land, however, and an unfortunate transformation occurs. In the time it takes to shower and throw on a hooded top, he'll turn from swaggering athletic hero into an awkward, troubled recluse.
Sometimes, Marzo will sidestep autograph-seeking fans, rubbing his hands, or anxiously pulling at his hair. Other times, he might give a withdrawn TV interview that redefines the meaning of monosyllabic. Occasionally, he can even turn aggressive, or come across as just plain rude.
In truth, he's no such thing. Instead, Clay Marzo, who exploded on to the scene at Puerto Escondido in Mexico last month with his first pro victory, and is now being dubbed the most naturally gifted surfer of his generation, has a high-functioning form of autism called Asperger's syndrome.
The condition makes it difficult for the 20-year-old wunderkind to interact with other people. He finds crowds unnerving, conversation perplexing, and is unable to "read" emotion on someone's face. In social situations he is often – if you'll pardon the expression – a fish out of water.
Yet Asperger's is also the secret of Clay Marzo's incredible sporting ability. According to an acclaimed documentary called Just Add Water, it actually helps him succeed in the glamorous field of professional surfing. He is a unique talent because of the condition, rather than despite it. Some believe it could eventually make him the greatest surfer in history.
"There's no one of his age in the world right now who does what he does," says Kelly Slater, the legendary, nine-times world surfing champion. "When I first saw him, I remember thinking, 'This kid knows things I don't know.'... He knows things all the guys I'm surfing with don't know."
Some people with Asperger's are genius mathematicians, scientists or musicians (many believe that Albert Einstein, Mozart and Isaac Newton had it). Others have brilliant memories, such as Dustin Hoffman's character in the film Rain Man. Marzo may very well be the first person with the condition to have developed a genius for surfing.
In Just Add Water, which will be released in major stores and on iTunes next month, one of the world's leading specialists on the condition, Dr Tony Attwood, explains how it has left Marzo with a unique ability to be able to predict how a breaking wave will evolve, and ride it accordingly.
"In Asperger's, the brain is wired differently," he says. "Some things are processed so superbly that the person has areas of excellence. There's a tendency to have what we call a special interest. It means that when the person with Asperger's chooses to do something, they will become an expert at it, probably one of the best in the world at it."
As the film's extraordinary footage of Marzo cutting up famous surf breaks in France, Tahiti, Australia and California shows, Marzo has harnessed his amazing memory (he can quote every word of the films Harry and the Hendersons and Elf) to compare every approaching wave, with other, similar waves he has encountered in the past.
"He'll have a schema of many waves that he's ridden before, so he's able to predict what to do in that situation," adds Dr Attwood. "His brain disconnects from everyday functions, and becomes one with the wave: he'll intuitively know what the wave's doing, so he'll anticipate that, and be ahead of everyone else."
Marzo's style of surfing is creative and spontaneous. Unlike most professional surfers, he doesn't plan different manoeuvres in advance, but instead adapts instinctively, suddenly spinning his board into reverse, perhaps, or flipping into mid-air to land audacious 360-degree turns.
The result is a uniquely extravagant style that marks him out from his peers. Having been blessed with a hefty dollop of physical talent, he can pull off even the most ambitious manoeuvres, sometimes landing a jump when a "wipeout" looks inevitable, or returning to an upright position after falling on to his back. Other surfers joke that he boasts double-jointed knees.
"Clay's an incredibly creative aerialist, and has this flexible thing in his legs that means he looks super loose and gets an incredible amount of velocity," says Just Add Water's director, Jamie Tierney. "Everything he does is over-rotated, but he's also got a real nice flowing style."
"He remembers absolutely everything. He can describe a wave to you from four years ago, just like that, and tell you everything about it. In the way someone like Mozart hears music in a different way to everyone else, he does that with a wave. He's one of the best young surfers we've ever seen. Period."
Marzo's career hasn't always been plain sailing, though. Before he was formally diagnosed with Asperger's, at the relatively late age of 18, his eccentric behaviour had prevented him reaching anything like his full potential in the sport.
A lifelong surfer, he had been born in Maui, one of Hawaii's larger islands, and brought up 30 yards from a beach. He got his first body-board at the age of two, graduated to stand-up surfing at four, and entered his first contest at five.
By 11, he had signed pro contracts with Quiksilver, and at 15 won one of his two national amateur championships, going into the history books as the first junior ever to record two perfect 10 scores in the final.
However, out of the water, things soon went downhill. Though clearly talented he hated the wetsuit modelling, magazine photoshoots and interviews that come with being a professional surfer. His difficulty in holding regular conversations made it tricky for him to charm fans at autograph-signing sessions, and other meet-and-greets.
He didn't understand the concept of marketing himself (one sponsor dropped him after growing tired of erratic behaviour) and fell out with some peers because of what they regarded as his anti-social behaviour.
More troublingly, Marzo's undiagnosed Asperger's also hindered his performance in professional surfing contests, since it made him tactically naïve. Competing in events that are strictly timed to last 30 minutes, for example, he wouldn't wear a watch, and would often fail to "hustle" competitors for the best waves.
Sometimes, when needing just a low-scoring ride to guarantee victory in an event, he would instead go for a recklessly ambitious one, fall over, and end up failing to post any score at all.
"Clay doesn't like being on a crowded beach when a contest is happening, so he won't watch conditions, and he didn't always do as well as he should," adds Tierney. "I remember one time when to win a heat all he needed to do was to score a 'one,' which basically means standing up on your board and riding it to the beach. But instead, he hung around trying to do something better and the contest just ended. He lost."
For a time, Marzo became troubled. He quit school, withdrew from formal competitions, and devoted himself to "free-surfing," travelling with camera crews to distant breaks, where he could be filmed doing what he loves best, away from the prying eyes of judges.
In one such project, two years ago, Tierney was asked by Quiksilver to make a surf film called Misunderstood. After they'd met, he realised that Marzo might have Asperger's (both Tierney's parents are psychologists) and persuaded him to visit a specialist treatment centre.
Marzo was formally diagnosed in December 2007 and has since been undergoing fortnightly therapy. He recently eased his way back on to the competitive scene and destroyed his rivals in Puerto Escondido last month. He will spend this weekend trying to repeat the trick in Bali.
Elsewhere in the film, which swiftly had its title changed to Just Add Water, Tierney deftly explains how diagnosis has allowed Marzo to finally understand and feel at ease with himself. It has also helped others get to grips with his personality.
"The main treatment for Aspergers is self-understanding and self-acceptance," adds Dr Attwood. "The problem is that people will talk of suffering from it. No. You don't suffer from Aspergers, you suffer from other people. I want people like Clay to be admired, for people to say they've got talent. Don't feel sorry for them, applaud them."
If he fulfils even half of his enormous potential, applause could be one thing that Clay Marzo ends up hearing an awful lot of.