UNDERTOW by Lynn Stegner (Baskerville Publishers: $21; 367 pp.). This stately first novel is about Anne McBain, a marine biologist caught in an affair with Elliot, a professional colleague who seems unable to extricate himself from his unhappy marriage. Even Anne's unexpected pregnancy fails to jar him into action. The one who moves is Anne, after her father's death forces her to confront the abusive relationship she had with him. Only after she sends herself into exile, studying whales, can she begin to sort out her life--and when she's done she emerges, sadder but wiser, to begin again. In synopsis, the story sounds distressingly like soap opera, but Stegner has dressed it in the most dignified manner. This is the story of anguish among the intellectual elite, and if its somewhat distant exchanges between Anne and Elliot frustrate from time to time, they have an elegant power, an integrity of purpose. It is almost as though Stegner is trying to protect her main character from any more distress, given the pain she endured as a teen-ager.