"You never know what the sea will cough up," observes an unwitting victim in "The Island." That goes double for the best-seller list.

Producers Richard Zanuck and David Brown may have found Peter Benchley's latest maritime thriller irresistible after the great success they enjoyed with "Jaws." But their reputations would have been safer if they'd tossed "The Island," a peculiarly loathsome piece of literary bait, back in the pre-publication pool.

Has Benchley acquired such a sacred commercial reputation that movie people now fail to recognize when he might be mixing ingredients likely to prove indigestible or unpresentable on the screen? The perfection of a mechanical shark seems a simple problem compared to suspending disbelief about the plot of "The Island." It borrows motifs from "The Land That Time Forgot," "The Most Dangerous, Game" and "Lord of the Flies" while spinning a fantastically vicious yarn about captivity among a band of atavistic pirates.

Michael Caine, another bad-luck star recently ("The Swarm," "Beyond the Poseidon Adventure" and now "The Island"), plays the typical Benchley protagonist, a civilized gent compelled to defend himself against brute force.

A magazine writer with the tony name of Blair Maynard, interested in writing a story about the disapearance of hundreds of pleasure boats in the Caribbean, Caine flies down to Miami for a weekend of nosing around. He is accompanied by his 12-year-old son Justin (Jeffrey Frank) who is footloose because his mother, the ex-Mrs. Maynard, is taking a holiday with a new boyfriend.

Following the structure of his novel, Benchley's screenplay alternates scenes of the Maynards nearing the danger zone with scenes of unwary yachters being murdered gruesomely by bloodthirsty night infiltrators of some kind. A fly-by-night charter pilot takes father and son from Miami to the islands, where the plane crashes and leaves them temporarily stranded. While passing the afternoon with a fishing trip, the Maynards themselves fall into a trap and end up captives.

Upon recovering consciousness, they find themselves at the mercy of a gang of cutthroats that has sustained a 300-year-old tradition of piracy. Hidden on a remote island, the scurvy crew, led by David Warner, assaults unsuspecting boaters, loots the vessels and endeavors to keep all the old barbaric laws and customs, which are codified in a yellowing "covenant" and interpreted by a resident historian.

Three centuries in seclusion have somewhat enfeebled the gene pool at this nasty Brigadoon, so the Maynards have been spared to infuse some desperately needed fresh blood. The historian, who also doubles as a brainwasher, soon has young Justin acting like a perfect little knave who might snuff his dad if the whim took him. Warner is so pleased with the lad's preposterously rapid progress that he wants to adopt him.

Meanwhile, the nanacled Caine has been condemned to stud service with the only female pirate believed to remain both fertile and undiseased. A gangly, rawboned Australian actress, Angela Punch McGregor, gets this singularly unattractive role, making her first appearance with her face obscured in mud at a tribal town meeting. Her second may be even sillier: She prepares the injured Caine for their wedding night, removes her burlap sack and settles down on the hapless journalist.

The nominal tension created by director Michael Ritchie during the preparatory sequences is broken with a ludicrous thud once the Maynards take up residence in Piratetown. This esoteric lost culture simply defies credible depiction. The sight of Caine being led around on a leash by his scrawny Moonbean McSwine, the boy's instant conversion to villainy, an often incomprehensible local dialect and the antics of make-believe cutthroats like character actor Dudley Sutton -- who cavorts around with his front teeth blackened and his hair styled in wigged-out cornrows -- combine to produce an illusion that is demonstrably laughable.

Nevertheless, before Caine can escape and be manuevered into a position to mow down his tormenters and also rescue his wayward boy (who does another 180-degree switch at the climax, presumably wiping the slate of earlier deviltry, which included shooting a man in cold blood), the buccaneers are permitted to divert us when assaults on a schooner and a Coast Guard cutter that produce plenty of freshly hacked victims.

This afternoon between the absurd and the gruesome is not deftly modulated, to put it kindly. In fact, it reduces your opinion of Ritchie and Benchley and everyone else involved to a level usually reserved for the sleaziest hacks in the low-budget horror market.

"The Island" might recommend itself to an evil-minded programmer planning the entertainment at Father Son smokers, but its suitability in ordinary moviegoing company is highly suspect. Did Ritchie and Benchley regard this stinko mixture of the goofy and the vicious as a droll effort? Some fun.