"Piranha 3D" is a genre horror film that delivers exactly what you'd expect: gushers of blood, ripped flesh by the kilo, and acres of bare booty and boobs (some of them real). But the movie has been fashioned at a surprisingly high level: The effects are top notch, the camerawork is marvelously fluid, and the 3D feels real, not like a post-production add-on (although it is). The thrills here may be traditional, but they're still actually thrilling.

The picture rips off "Jaws" with vigorous glee. In the part of the local lawman played by Roy Scheider in the 1975 classic we now have Elizabeth Shue, and Adam Scott takes the part of the science guy played in the earlier film by Richard Dreyfus. (Dreyfuss himself actually pops up here, in a very small part that requires him to be rendered into even smaller parts.)

The setting is Lake Havasu, Arizona (here called Lake Victoria), where a seismic event of some sort has opened a subaqueous crack, up through which pour a legion of primordial piranhas ("more than two million years old!" one knowledgeable character gasps). And wouldn't you know, this ominous event has occurred right in the middle of the annual spring-break invasion of drunken college boys and knockout bikini women. (There are lingering shots of the inevitable wet-t-shirt contest.)

The story is unabashedly by-the-numbers. While the college kids frolic on the lakeshore, a creep named Derrick (Jerry O'Connell) is out on the water in his cabin cruiser with local boy Jake Forester (Steven R. McQueen), son of the sheriff (Shue); nice-girl Kelly (Jessica Szohr), on whom Jake has his eye; and two bisexual babes (Kelly Brook and Riley Steele) who swim around naked beneath the glass-bottom boat so that Derek can film them for a skin flick he's making. There are also two little kids stranded on an island and wading out knee-deep to shout for help. All of this solicits the traditional audience response: "Don't go in the water!" But does anyone listen? Need you ask?

Director Alexandre Aja ("High Tension") knows there's no point in using 3D for subtle depth effects; what we really want is vicious multi-fanged piranhas shooting right off the screen and into our face. He does this with great gusto; and working with cinematographer John R. Leonetti, he's concocted some ripping subsurface scenes (beyond the pull-'em-under shots familiar from "Jaws") and some lively stunts. We expect Sheriff Forester to start running around on the beach in a panic pleading with the college kids to get out of the lake (to no avail until the water is running red with blood), and so she does. And when Derek's boat founders on some rocks with one of the girls trapped below deck, we know that somebody has to jump in the water and swim below to rescue her. We know, we know. But when the action is this slickly done, we want to see it all again, one more time.