"Hast seen the white whale?” a Melville-loving officer aboard a research vessel asks Donovan Hohn, in his dazzling “Moby-Duck,” whenever they pass in the ship’s corridor. “Hast seen the yellow duck?” Hohn cheerfully responds. The answer is always no, but this hardly dampens Hohn’s enthusiasm for his Moby — a load of bath toys that plummeted off a storm-wracked container ship in the northern Pacific in 1992. The maritime misfortune was exciting for beachcombers, who would find the toys on North American coastlines for years to come, and it provided data for scientists who study ocean currents. It also spurred the map-loving Hohn, a dozen years on, to give up his Manhattan teaching gig and embark upon what could have been a fairly straightforward investigation. Where did the ducks come from, where did they drift, and why? MOBY-DUCK The True Story of 28,800 Bath Toys Lost at Sea and of the Beachcombers, Oceanographers, Environmentalists, and Fools, Including the Author, Who Went in Search of Them By Donovan Hohn 402 pp. Viking. $27.95.