With three novels now availble as evidence, it is clear that Peter Benchley writes according to a formula. The formula, moreover, is a simple one: take a lot of salt water and put into it something unexpected and menacing. Anybody can do it, and in the wake of "Jaws" quite a few have tried. The problem (the writers' problem, not the reader's) is that nobody seems to do it quite as well as Benchley.

For one thing, he is more imaginative than other writers about what he puts into his water. While others were trying to rewrite "Jaws" (and some actually had their efforts published), Benchley introduced a new theme, that of sunken treasure, in "The Deep." Now, for his third oceanic experience, he has come up with a theme as time-worn as the two others, and once again he has given is a salty new life.

This time the theme is pirates, and anyone who has read "Treasure Island" or "A High Wind in Jamaica" will feel at once that the terrain is familiar, the ingredients tried and true. But despite occasionally disturbing evocative echoes, Benchley had once again made the story his own. Anyone who enjoys a good adventure yarn will race through "The Island," and those who take it to the beach (Benchley has timed his publication to make that likely) will find that nature has provided appropriate atmosphere and sound effects.

Pirate stories are, of course, a standard commodity in the paperback trade: they are period pieces, they have a buxom wench at the center of the action and their basic motif is the inordinate lust a man generates in all those lonely months of striding the poop deck, looking for a vulnerable sail on the horizon.

Benchley brushes sex aside and harkens back to an older and, on the whole, a more interesting type of pirate story - the type that involves a child rather than a woman. He also abandons the traditional 17th-century setting. His pirate are modern, or rather they are 17th-century pirates surviving and operating in modern times. When you stop and think about it, it seems rather improbable, although no more so than his two previous sea adventures.

This time, Benchley's central characters are Blair Maynard, "Trends" editor for a weekly newsmagazine, Today, and his 12-year-old son Justin. One trend Maynard notices is the tendency of boats to disappear in a certain small area of the Caribbean (610 in three years), and without any particular encouragement from his employers he decides to go off (taking Justin along) and investigate. He is obviously an excellent investigative reporter; almost immediately he not only discovers the pirates but becomes their prisoner - again taking Justin along.

The pirates are a scruffy but fascinating lot, descendants of a band of buccaneers who took refuge on an inaccessible island in 1671 and since have eked out a precarious existence, fighting constantly against malnutrition, a hereditary venereal disease and (perhaps most menacing of all) the group's own internecine violence.

The Book's central question is two fold: will Maynard be able to escape from the island, and will he be able to retain the allegiance of Justin, who finds the buccaneer life stimulating after the humdrum existence if a schoolboy in New York City, and who shows promise of becoming l'Ollonois in the next generation?

It's a close call; Justin participates in some of the novel's copious blood-letting, and for a while it is like a game. But with the realization that his own blood, too, might be let, he becomes a frightened little boy.

Those who have read Benchley's two previous novels of adventure at sea will find his writing continues to improve. The plotting is tight and intricate: the exotic scenery (including some very exotic people) is well integrated into the story; the basic implausibility is well palliated, and the reader is kept wondering what will happen next until the last page. "The Island" probably will not make as big a splash as "Jaws," but in many ways it is a better book.