Is it a strange mammal related to the seals, a descendant of a prehistoric reptile, or a new, unidentified animal? Whatever it is, or was, the witnesses call it a sea serpent. Remarkably similar descriptions of a creature with a long body, undulating motion, and horse-sized, snake-like head have left a trail of clues and controversy going back three centuries. In "The Great New England Sea Serpent," J.P. O'Neill draws on the historical record as well as previously unpublished first-hand accounts to chronicle more than 230 sightings of the mysterious marine creatures inhabiting the Gulf of...