SEA MARKS by Gardner McKay; directed by George Martin; scenery by Robert D. Soule; lighting by John F. Custer; costumes by William Lane; slides by Peggy Hansen and Michael Guy.With Timothy Crowe and Mina Manente. At the Eisenhower Theater through June 7.

Colm Primrose is a fisherman who lives in a stone cottage in a place called "The Heads," somewhere on the Irish coast. He lives by himself, but he has not forgotten a woman from Liverpool he met at a wedding the winter before last. So he writes her a letter, and she writes back.

Those are the not-exactly-earthshaking beginnings of "Sea-Marks," which opened at the Eisenhower Theather last night after a spur-of-the-moment passage down the East Coast from Providence's Trinity Square Repertory Company. Thanks to two superior actors, Timothy Crowe and Mina Manente, and to author Gardiner McKay's genuine affection for the sea and anything connected with it, this is a livelier and more absorbing production than some with 10 times as many characters and plot complications. But like the sea, the theater is unpredictable, and "Sea Marks," after starting out small, simple and true, ends up small, simple and as phony as a waterlogged three-dollar bill.

For Colm the Fisherman becomes Colm the Best-Selling -- an "Irish primitive," the Robert Frost of the sea." And "Sea Marks" becomes the story of a man forced to choose between, on the one hand, love, Liverpool and the literary life, and, on the other (glug!), the Call of the Sea.

But that's Act Two. Back in Act One, when Colm is writing only letters, he writes of his low regard for the Catholic Church ("I don't work six days a week to be made small the seventh"), of the travails of winter fishing ("I'll tell you how cold it gets -- sometimes you cannot speak, that's how, cold it gets") and of the Irish national penchant for knitting ("If the winter were long enough, I could knit a house").

Won over by his letters, Timothea, his correspondent, comes to The Heads for a short visit. Then Colm goes to Liverpool for a longer visit, and he is invited to share Timothea's narrow bed. This causes him to blurt out that "What you're talking to here is a 35-year-old spinsterman!" And when she starts to caress his leg, he jumps up and begins an evasive discourse on sleeping whales. "God, there's a sight.

It is after they have become lovers that Timothea reveals her little secret: She and the publisher for whom she works have assembled a series of snippets from Colm's letters into a book called "Sea Sonnets." Let us, for the moment, slide past the plausibility (in this litigious age) of a publisher's deciding to print a book without bothering to consult the author. The larger question here is why anyone would want to publish this book at all. Why do we need a Robert Frost of the Irish Coast to tell us that fishing is like "sea farming" and that the sea is "a continent?"

Colm, to his credit, seems startled by the success of his literary efforts. Timothea expects him to settle down in Liverpool and go on writing. "Don't you want to touch people with your words?" she asks him.

"Do I know these people?" he asks back. Fishing is his craft, he says, and The Heads "the only place I know . . . I love you very much and I suppose I'm dying because of that love. It's a slow way to die, because I'm away from where I belong."

Crowe and Manente, both veteran members of the Trinity Square Rep, have rich, auditorium-filling voices that are a pleasure to listen to, and they put the same delicacy and conviction into the play's weakest passages as into the strongest.

As a result, "Sea Marks" holds up far longer than it might in other hands, and theatergoers who are fond of love stories and the sea (and who haven't been Irished-out by "Da") may find this play's virtues winning and its flaws inconsequential.

Whatever one thinks of "Sea Marks," however, the scenery by Robert D. Soule should be chopped up and shipped back to Rhode Island. The slides of Irish sky and Liverpool skyline do not at all give us a feeling of having moved from one locale to another. And the cut-off boards that frame Colm's cottage and Timothea's flat seem appropriate to neither setting. They are merely a trend in set design -- and, one hopes, a passing trend.