IN THE GEORGE PLIMPTON tradition, engaging in a calling bizarrely different from your usual job, should be a sure-fire formula for writing a successful book. The author's undeniable one-upmanship gives him instant authority since he has access to a chosen subculture for a behind-the-scehes account.

Randall Peffer, author of Watermen, chose his subculture, that of the Chesapeake Bay commercial fishermen, because he hoped to search for his ancestral roots in the Bay area. That search gets lost somewhere in the halyards, but after a year on the water, Peffer, who teaches at Phillips/Andover Academy, has given us an account of the colorful, encapsulated life-style of tough-talking, rough-living, warm-hearted fishermen in an area with precooked reader interest, thanks to James Michener's novel Chesapeake. (That recent best-seller is now excerpted in The Watermen, also reviewed below.)

Peffer settles on Tilghman Island (pop. 1500) off Maryland's Eastern Shore, and works under Captain Bart Murphy on the Ruby G. Ford, a skipjack, one of 30 oystering sailboats still working. "She's just a dumb old boat, eighty-six years this season, but she'll still catch a lick o' arsters," the captain says.

Randy, as the watermen come to call the new man in town, goes "drudging for arsters", learns about tongers who laboriously pluck these bivalves from the bottom, and about ways around the law, such as dredging from an old Simca through the ice. Randy, on the Ruby Ford , plants 1,000 bushels of spat (young oysters) a day for state pay. He fishes for eels, hunts geese, goes crabbing, clamming, crews for a bluefish charter. Throughout the year there is the presence of Captain Bart, his crew and various of his peers, ordering swearing joking, as the neophyte is inducted into the brotherhood of watermen.

Peffer learns that no waterman will go out in a boat painted blue, or if three crows fly across its bow. He describes the search for a lost crabber, and his funeral -- "the drawing together of this island community," the minister says. There's good fellowship on the wharves and in the boats as the men pursue the current catch, dodging the Marine Police or the Department of Natural Resources. Peffer's account of work on the water is thoroughly believable. The flavor of the job and the working fishermen's attitude toward it flows easily.

"Damn, boy, let's get movin'. This weather's goin' to get worse 'fore it moderates," says Bart Murphy at one point. And Peffer takes such instructions literally, but only as they apply to sailing. He doesn't get moving into the subject of the bay itself -- the breadth and depth of its deterioration and how that will affect the watermen's lives, present and future.

That was the big important story Peffer had in his net, but he didn't recognize it. The men themselves often hint at the larger themes of their lives; Peffer faithfully records such hints but lets them lie, unexplored, with no more or less attention than he gives to what's happening to the mainsail in a gale. Blue crabs, for example, were scarce in May. "Got to have more than five bushels to make a livin'," Bart says, as he pulls in only three, and other crabbers even less. Soon the watermen give up crabbing and go after eels. "Newsmen, seafood buyers and environmentalists began their favorite chant: the Chesapeake was dead," the author notes and with no further comment gets up in a red sunrise to go eeling.

There's a puzzling shortage of clams, causing illegal clamming in oyster beds. "Hurricane Agnes brought us to this sneakin' around. Killed all the grass on the bottom of the Bay," a waterman saus; another says, "Poor conservation's the bad guy." Peffer rests the case, as he does when an elderly resident tells him of how Tilghman Island's western shore has eroded, or how the Bay's water level has risen due to siltation.

Yet the change in the once-magnificent productive Bay and in its watermen's ability to continue their known ways is the guts of any meaningful record. Watermen needs a past and a future, needs to embroider the change its characters point to with the intelligent exploration the author might have applied to it. Peffer's straight and narrow course confines his book mostly to the fishermen's work, skirting the profound troubles it encounters. He deprives us of knowing the watermen's families, their homes, the sights, smells and social character of Tilghman Island in back of its wharves, all of which have a hand in producing these watermen.

Watermen comes out as flat as a tape-recording. But it whispers that when the whole story of the Bay and those who hunt and are hunted in it is told, it will be as suspenseful a tale as any to be found during these years in the still-blue Chesapeake waters.

WARNING: Do not confuse Watermen , which is at the very least factual, with another book with almost the same title, which is not. James Michener's The Watermen , son of Chesapeake , is no improvement on the parent. It passes up drama that is intensely interesting because it is real in favor of the author's special brand of fiction, using the Bay as background. The Watermen may touch on facts, but it is impossible to tell what is true and what is Michener. The selections rattle along about a goose named Onk-or, "married to his mate eternally," Jimmy the blue crab and his mate, about dove shoots, hunting, oyster wars, game wardens, guns.

This combination of children's nature stories and Sports Illustrat:d , is highlighted by pictures by John Moll, who has turned in some handsome representational pencil drawings of geese, retrieving dogs, ships under sail. But his illustrations of people resemble stiff-jointed dolls dressed for their parts as the Game Warden, the Captain, the Diving Man in Flames. These illustrations, Michener says in his introduction, are his reason for reissuing his hugely successful novel in a new guise.