He has dived into the subject again and again in his films - from his debut feature in 1981, the ultimate schlock-horror B-movie, Piranha Part Two: The Spawning (which he now laughingly calls "the finest flying piranha film ever made"), to the eerie deep-ocean fantasy, The Abyss, in 1989, and of course, the multi-Oscar-winning Titanic.
According to the director, who turns 51 next week, "beside film-making, the underwater world has always been my other love. So if I get an opportunity to be able to put the two together and to make a film on an underwater subject, then I can't be happier. If I had to choose one over the other, I would probably dive."
So he was very pleased to be offered the opportunity to combine his two great passions on his latest project, Last Mysteries of the Titanic, which is showing on the Discovery Channel on Saturday. This is a fly-on-the-wreck view of the Titanic, which lies two and a half icy miles beneath the surface of the Atlantic off the Grand Banks of Newfoundland. For the purposes of the programme, Cameron helms a flotilla of state-of-the-art research subs down to the stricken ship. The aim is to gain access, for the first time since the craft plummeted to the ocean floor, to what explorers see as the Titanic's two "holy grails": the ship's Turkish baths and its boiler room.
He says he relishes the sheer unpredictability of non-fiction. "Documentaries are hard," he asserts. "The kind of filming I had done before, where you have a script and you know what you're doing, is easy by comparison. When you're shooting a documentary, you never know whether you're wasting your time every time you start squirting off some footage, or whether this could be the moment of gold."
Cameron, who was born and bred in Kapuskasing, Canada, goes on to explain his love affair with documentaries about the deep. "When I was a kid, exploration was the most important thing. When I realised that I wasn't going to be an astronaut and I wasn't really going to go to other planets, I became very interested in the ocean.
"The imagery that Jacques Cousteau was putting on television back then in the mid-Sixties made me realise that there are alien worlds right here on Earth that you can explore for the cost of the Scuba equipment.
"I still have the same urge to explore and to understand the wonders of the natural world. Now I'm getting to live that fantasy." A fortune estimated to exceed $50m may well be helping him achieve that goal.
An imposing, 6'2" figure with a neatly clipped, greying, beard, the five-times-married Cameron bubbles with enthusiasm about the life aquatic. A self-confessed "nerd from Kapuskasing", he is utterly immersed in all things maritime. As he outlines in exhaustive detail the technological advances that have been made in submarine filming over the past few years, he breaks off for a moment to laugh: "I must warn you, I'm into this stuff."
Cameron first became intrigued more than a decade ago by the story of the Titanic, the grand liner that was launched in 1911 amid a blizzard of ticker-tape and hype. Less than a year later, at 2.20am on 15 April, 1912, its crew ignored all warnings of impending danger, and the ship struck an iceberg and sank. Of the 2,208 people on board, only 705 - predominantly women and children - survived.
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The director worked the story up into a $200m shipwreck epic which soon sailed into the record books as the highest-grossing movie of all time. It rang up an eye-watering $1.7bn at box offices around the globe.
Cameron returned to the subject two years ago when he piloted a sub down to the real wreck of the Titanic to make the 3-D documentary Ghosts of the Abyss. So why, all these years after his initial interest was pricked, is the film-maker still hooked on the story of the mighty liner that came to a mightily sticky end?
"I felt I'd finished with it after making Ghosts of the Abyss," Cameron concedes. "But a little voice in my head kept saying, 'You've only searched 30 per cent of the wreck.' And so I thought, 'This is unfinished business. We now have new smaller, more sophisticated vehicles. Let's finish the job and make the definitive archaeological survey.'"
But, more than that, Cameron emphasises that the Titanic has immense symbolic significance. "You have to start from the fact that the Titanic is different from all other shipwrecks," reflects the director, who has made several other marine documentaries, including Expedition: Bismarck, Volcanoes of the Deep Sea, and Aliens of the Deep.
"The Titanic has a great metaphorical and mythical value in the human consciousness. Is it the most compelling thing in the world when we need to find a cure for Aids and millions of people are dying in Africa? No, on that scale, it's not a priority. But you have to think of the Titanic in terms of a feature film or a novel - something that touches people's emotions. Wrecks are human stories. They teach us something about ourselves. A wreck is a fantastic window into the past. Steel can't lie - it doesn't have an agenda. These wrecks are like time-capsules. We'll put parking lots over battlefields, but underwater these sites are frozen in time. By visiting them, we can touch history."
So what does the Titanic have to teach us today? "People cluck and say it's not relevant because the class structure of that time doesn't exist anymore, but it really does. Contrast the way we in the West live with they way people live in, say, Africa or Indonesia. There is still first class and there is still third class. We're all living on one big blue spherical Titanic." Cameron continues that there are also lessons to be gleaned from the way the ship came to grief. "Like the crew of the Titanic, we've identified the icebergs, but we're not reacting quickly enough as we approach them. By the time they reacted to the icebergs, their fate was already sealed. That's a great metaphor for today. Think about global climate-change. By the time we see evidence of it, it will be too late - a collision will inevitably occur. Mr Bush might have some questions to answer about that."
The director gives another example of what we can learn from deep-sea treasure-troves. "Look at the wreck of the Bismarck, the Nazi ship that I explored a couple of years ago. That opens a window onto a specific time in history. It gives us an insight into a certain mindset and makes it more immediate. A lot of kids watched Expedition: Bismarck, and all of a sudden to them the Second World War became more real.
"It's a way for me to give something back, in a sense, and not just be a taker, who just makes films and makes a lot of money, because ultimately that doesn't really return anything other than entertainment value. I don't want to negate that, but I think there's so much else that can be done.''
He is awestruck by the often unheralded endeavours of scientific researchers. "I identify with them. They're basically people who don't live in a glamorous world. They live off the beaten path and spend a lot of time on ships at sea. They're, in a sense, cloistered in academia, but they're really heroes because they're at the cutting edge of human exploration. They're at the frontier of knowledge."
He believes that the work of such pioneers underlines the shallowness of our celebrity-fixated society. "Most people are involved in making money. Unfortunately, in our society you are seen as a chump if you don't do that. People who pursue other dreams are the ones who interest me most, whether they are artists, explorers, writers, scientists, or people looking for some greater meaning or other purpose. I think these are the only people worth knowing and worth celebrating.
"Unfortunately, our Western society tends to celebrate the wrong people, people who entertain us in a very superficial way but don't entertain us intellectually. I don't have any problem with those folks, I just don't think that they should be put on a pedestal." After winning 11 Oscars for Titanic in 1997, Cameron was himself put on a pedestal by Hollywood moguls. He could have named his movie - and his price - but elected not to repeat himself with endless clones of his greatest hits.
Finally, though, he thinks the time is right for him to return to feature films because he can now harness new technology to make something entirely fresh. Unsurprisingly, he has opted for an almost insanely ambitious sci-fi blockbuster. It is clear that the director of such ground-breaking films as The Terminator and Terminator 2, Aliens, The Abyss, and True Lies wants his comeback movie to make as big a splash as they did.
Inspired by Japanese graphic novels, he is currently developing Battle Angel, a cyborg thriller set in the 26th century. "It's going to be a mega-budget film shot in 3-D," Cameron enthuses. "It's set in a post-human world in the distant future, and a number of the main characters will be computer-generated. It's a kind of virtual film-making. We're building a whole new motion-capture technology. I'm impatient to get on with using the tools of the future."
He continues: "The main thrust is a love story between a human man and a female cyborg, and the film contains a range of characters from the fully human to the fully machine. I'm embracing the fact that human beings are amazingly adaptable. We've got a lot of flaws, but we're also pretty clever. We've got the tools, but can we use them?"
So does this return to movie-making indicate that Cameron has finally got the Titanic out of his system? He reckons so. The director, who was reportedly at the head of the queue to pay $200,000 to go on Virgin's inaugural commercial space mission, says that "with Last Mysteries of the Titanic, I'm hoping we'll able to lay a few questions to rest. I've made the decision not to return anymore. We've shed a lot of light on it now, and enough's enough.It's time to move on.
That does not mean, however, that Cameron will stop being fascinated by this gigantic hulk of metal that has lain rusting on the ocean bed for almost a century. As far as he's concerned, the Titanic spell has not yet been broken.
"Over the years," Cameron muses, "I've found the Titanic story to be a wonderfully rich and renewable metaphor for the way we look at the world. I'm afraid that human nature has not changed much since 1912 - if at all!"
'Last Mysteries of the Titanic' shows on Saturday at 9pm on the Discovery Channel
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