TITANIC (12) is one of the most spectacular films ever made. It's also one of the most badly written. And yet, despite the abyss between James Cameron's meagre screenwriting talents and the apocalyptic grandeur of his direction, Titanic stays afloat. The dialogue may be unspeakable, but the film remains unsinkable.

With 100 minutes of build-up before its 60,000-ton star begins to founder, Cameron has to elevate his passengers above the status of iceberg-fodder. So he pairs poor little rich girl Rose (Kate Winslet) with penniless artist Jack (Leonardo DiCaprio), and anchors his melodrama to a romance between peerage and steerage. To this he adds a modern framing story, in which a 101-year-old Rose (Gloria Stuart) returns to the scene of waterlogged hubris, where fortune hunter Lovett (Bill Paxton) is diving for diamonds.

The first hazard that Titanic has to negotiate is the physical disparity between its stars. Unlike some of her anorexic Hollywood colleagues, Kate Winslet has pint-pulling arms and fetlocks like a carthorse. She also has enough sassy authority to make a terrible line like "So you think a first-class girl can't drink?" sound almost convincing. Unfortunately, DiCaprico is tiny in comparison, and - despite that sleazy smile that makes you think he'd go down on you for a tenner - he looks like he should be reciting his times tables to his co-star, not hoiking her from fo'c'sle to poopdeck.

And whose idea was it to make them connoisseurs of fine art? With precognitive taste for the masterpieces of the future, Rose has nothing in her luggage but Picasso, Degas and Monet - over whose water lilies she and Jack fall in love. Their analysis would be enough to make Sister Wendy throw in the wimple. "Look at his use of colour here, isn't it great?" enthuses DiCaprio. "Yes," returns Winslet. "Incredible, isn't it?" And before you can say La Giaconda, Leonardo is sketching her in the buff - though the drawing he produces is more like something a Pittsburgher would airbrush upon his Chevy than the work of a Post-Impressionist draughtsman who's supposed to know his art from his elbow.

But it's not as if other areas of the movie are overburdened with realism. Cameron treats the disaster as myth or metaphor rather than CNN-style actualite, and Titanic is only social history to the extent that Carry on up the Khyber is a document of the Indian Mutiny. The astounding digital effects allow the ship to pitch and split and slide with sickening dynamism, but this flexibility also robs it of its destiny as a real object: on the first leg of its journey, New Age dolphins swim by the prow; later, as the shipboard romance warms up, the deck is bathed in honeyed light, turning the screen into an animated Edwardian travel poster; in the dark aftermath, we're shown the Stygian image of frozen corpses bobbing in a silent sea.

So what is this myth about? What is it a metaphor for? Armageddon? The millennial bomb? The fall of capitalism? "To me it was a slave ship," says Rose, sticking to soft-centred Marxism as Cameron releases a deluge of allusions to momentous political events: a diamond pendant is a relic of the French Revolution; the caddish Cal Hockley (Billy Zane) falls foul of the Wall Street crash; Lovett's sidekick compares Rose to "that Russian babe, Anaesthesia."

But Titanic isn't Battleship Potemkin. Though Cameron advocates the multi-ethnic cheeriness of the steerage passengers over the Arctic snobbery of their betters, his film's emotional impact derives almost entirely from unreconstructed stoicism of the stiff-upper-lip variety. We see Benjamin Guggenheim (Michael Ensign) sipping brandy, watching the water lap around his chair. We see Captain Smith (Bernard Hill) alone on the submerged bridge, water straining against the windows. Most movingly, we see Officer Murdoch (Ewan Stewart) shooting himself in the head, appalled that he's just gunned down a passenger in the chaos of the lifeboat queue. The spectacle of the upper classes drowing in their diamonds is hypnotic, and Cameron can't suppress his nostalgia for the plutocrats he sends down the plughole of history. Since he's just made the most expensive move in the history of cinema, perhaps that's unsurprising. MS