The remains of the Titanic look as if they're literally melting into the past. Gray and corroded and draped with ''rusticles,'' the ship's murky ruins are spookily preserved in the North Atlantic, even as the ocean water gradually eats them away. This awesome wreck is a majestic subject for the 3-D IMAX Ghosts of the Abyss, and James Cameron, who produced and directed it, does a visually splendid job, though what he has fashioned comes down to a logistical footnote to his great, primal, heart-of-the-ocean blockbuster. Cameron and one of his ''Titanic'' cast members, Bill Paxton, who looks nervous enough to lose his breakfast (and eventually does), descend in a pair of nifty, Bondesque mini-subs to survey the gargantuan hulk, each section of which gets matched up to photographs and computerized images of its pre-iceberg splendor. There's an amusing glimmer of suspense generated by the film's twin robot cameras, which roam the ship on fiber-optic snakes. Will the cameras make it through the exploration intact? Cameron wants to take the audience ''back to 'Titanic,''' but the journey's magic is hemmed in, paradoxically, by the transcendence of his previous effort; surely he must know that a lot of us never left.