I have seen the future and it smells fishy. But first, the good news: Hollywood has finally disappeared under the oceans. Welcome to Waterworld, the new Kevin Costner vehicle, set in a post-apocalyptic tomorrow where global warming has melted the ice-caps and we are cast adrift on an endless expanse of briny blue.

Waterworld begins with Kevin pissing into a bottle. In this future, you see, salt-free fluids of the bodily kind are precious. So Kevin recycles his wee, pouring it into a makeshift filter, and drinks it. A reference to extracting urine is a risky way to open any movie, but especially so Waterworld, which at a final cost of more than $200 million is easily the most expensive film ever made and, inevitably, one vast aquatic turkey, neither fish nor fowl.

And yet, despite the fact that Costner's profligacy shames us all - think how many Bosnian refugees could be protected, fed, clothed and sheltered with $200 million - despite the breathtaking arrogance of this imbecilic gesture, I do believe that Kevin Costner has finally forged a truly modern myth, a tale of our time, symbolic and visionary in its nature. The only problem is, he almost certainly didn't mean to.

Waterworld is a terrible place (what did you expect, with America gone?), where lonely Drifters on driftwood boats search for other Drifters to trade with, or rip off. They are easy prey for the Smokers, greasy Hell's Angel types led by Dennis Hopper, who maraud the high seas on jet skis, pillaging, raping and looting. The reason we know they're bad guys is that they burn oil and - wait for it - smoke cigarettes.

Into this sorry, soggy mess sails Our Kev on his oil-can catamaran, grimy and tan, the mariner's Mad Max, the American outlaw loner, free to roam the open seas in his watertight Cadillac. Naturally, although limited to environment-friendly sail-power, his boat has the biggest mizzenmast you've ever seen, and performs like some sea-bound Batmobile. This Bobbin' Hood has a dark secret: he can swim like a marlin because he's a mutant, with gills behind his ears and webbed feet. He's packing some blubber, too. (Given a moustache, no female walrus could resist him.)

Pretty soon he takes on board love interest Jeanne Tripplehorn and her strange charge, a sassy nymphet whose back is tattooed with a map showing the way to the legendary Dryland. Kev hates the little girl, who sings and scribbles on his deckhead. "Don't draw on my boat!" he rages, threatening to throw her overboard. "Shut up. Sit still. Don't touch anything!"

When the females disobey, Kev hacks off their hair with his machete. This symbolic violation has a strikingly domestic tone, because here Costner is appealing directly to his core audience - the Angry White Males, the survivalists, the right-wing libertarians who, driven to paranoia by the waning of their cultural dominance, vent impotent rage through terrorist acts like the bombing of the Oklahoma federal building. But Kev wants his fish-cake and to eat it, too. He plays the eco-card, and he is a thinker. "Nothing is free in Waterworld," he sneers. Nothing, that is, except his duck-billed platitudes.

Waterworld is a queer fish, in which only Hopper, as a maritime Mussolini with extra prosciutto, manages to convey any semblance of character. (In fact, Costner should get on his knees and fellate him for keeping the entire movie buoyant long after it was dead in the water.) Amid innumerable explosions, Kev fumbles towards a stunted relationship with the little girl, while all but ignoring the conspicuous mammary-jogging of Tripplehorn. Instead, we see a ritualised rapprochement between "father" and "daughter", the Frontier Spirit and the New Age, the Cowboy and the Inner Child, as they swim underwater in slow-mo.

Possession of the girl provides the showdown between Costner and Hopper, and the pyrotechnical climax is sheer James Bond, a load of old fireballs. (It is alleged that the filming of Waterworld caused ecological damage to Hawaiian coastal waters.) Ironically, despite his stand against the nominal forces of evil, there is no discernible difference between Costner and his foes. Both do as they please, take what they want, and batter anything that confronts them into submission.

But Costner redeems himself with a finale of unwitting genius. On reaching Dryland (soon to be a Disney theme-park) everyone is ecstatic except Kev. Being the macho outlaw loner, he cannot tolerate a prissy, land-lubbing existence, and bids tearful farewell to all, sailing off into the sunset.

And then you realise. Kevin, the Angry White Male survivalist, has already mutated, and out at sea the process will continue. At best he will fail to procreate and die. At worst, devolution will eventually return him and his ilk to the true primal state, that of a single-celled amoeba. Adieu, pond-life.

The rest of us, meanwhile, can get on with rebuilding the world around the songs and drawings of little girls. Now that's what I call a happy ending