The sea might be deep in “Into the Blue,” but everything else about the slick but silly adventure flick is as shallow as can be. If the idea of ogling Jessica Alba as she swims through turquoise Caribbean waters in an ultra-skimpy bikini appeals to you, or, alternately, of watching Paul Walker show off his pecs doing likewise (though in a regular, manly suit, of course), this glossy but brain-numbingly dumb movie might afford you some pleasure. Anyone else is likely to find the view a lot less appealing. This is a soggy mixture of treasure-hunt caper and drug-smuggling thriller, as narratively (and visually) murky toward the close as the ocean depths are clear at the beginning. And its characters are a thoroughly cardboard lot. Where’s Bruce the Shark when we really need him?

Matt Johnson’s script introduces Jared Cole (Walker) and Samantha “Sam” Nicholson (Alba) as a lovey-dovey couple living cheap in the Bahamas–he’s (for a few minutes at least, before chucking the job) a part-time member of a tour crew, but his real interest is in searching the ocean floor for old shipwrecks and the loot they contain. She’s a shark wrangler at a seaside park whose main interest is Jared, and they live together in a run-down trailer where he spends a good deal of his time trying to get his derelict boat back into condition to sail. The duo’s situation changes with the arrival of Jared’s pal Bryce Dunn (Scott Caan), a criminal defense attorney who comes for a visit trailed by his newest squeeze Amanda Collins (Ashley Scott), a striking blonde with, as it turns out, a drug habit. Bryce takes the group to the posh mansion his legal work has acquired for him, at least temporarily, and they all go out on the boat that comes along with it to frolic in the deep. It takes them no time at all to discover not only a few items from what might be an exciting (and valuable) old wreck but the remnants of a crashed plane that happens to contain a huge cache of cocaine. Bryce and Collins eye the drugs greedily, but straight-arrow Jared and Sam are concerned only with discovering the ship from which the treasures came. Unfortunately, pressure from the efforts of scumbag salvager Derek Bates (Josh Brolin) to beat Jared to the find eventually lead Bryce and Allison to take some of the stash from the plane to finance a real expedition, and when their plan goes awry, Jared is sucked into their scheme in order to save them, and before long one of the group is dead and the others threatened by the drug dealers whose plane they’d found. The last act features double-dealing, chases on sea and land, some damsel-in-distress hokum and complicated rescue razzmatazz, but it’s all so confusingly staged and sloppily shot that it’s frequently difficult to tell what’s going on. And there’s still that fabulous shipwreck on the bottom to deal with.

“Into the Blue” is juvenile stuff, no more advanced from a plot standpoint than old Saturday-matinee serial fare. And the cast performs at about the level of those old chapter pieces, too. Caan is the most irritating of the bunch as the motor-mouth instigator of the scheme that puts everybody in danger, but Scott comes in a close second as his dippy, amoral partner in slime. As for Walker and Alba, both are easy on the eye and know how to pose both above and below the waves, but that’s about all they have to offer. Certainly their delivery of the lame dialogue leaves a good deal to be desired; a particularly memorable moment comes when Alba tries to deliver a line describing Amanda as a “coke whore,” with hilarious results. (It might even make you glad that later on, Alba has to spend a portion of the picture bound and–more importantly–gagged.) But master actors couldn’t have done much with this material: Jared and Sam are mere sketches rather than people, and it’s impossible to raise the faintest hint of concern or suspense about their fate. Brolin’s scowling villain is equally bland. Director John Stockwell, with the complicity of D.P. Shane Hurlbut, brings the same brand of splashiness he did to “Blue Crush” to this equally watery piece, but in the later reels the crystalline visuals of the opening sequences are jettisoned in favor of dark, gloomy underwater images. That’s probably an intentional shift in line with the change in narrative tone, but it doesn’t make the closing stages of the movie any easier to watch.

Filled with skin, sun and rolling waves, “Into the Blue” has a travel-brochure look that might afford some momentarily decent postage-card stills, but beneath the flashy surface it doesn’t have a shred of intelligence, and it sinks in a sea of lame writing and laughable performances. An exclamation one of its handsome but unlikable characters makes about another fits the movie as a whole: “Stupid, so stupid!”