WHEN NOT TEACHING government at Harvard, Professor James Q. Wilson can often be found -- even if not often recognized -- wearing self-contained underwater breathing apparatus (scuba) and bubbling above coral reefs. His wife Roberta is his constant diving companion, and this admirable book suggests that their excursions are as productive as they must be enjoyable. Billed as "an easily understood summary of what scientists have discovered about life on the coral reef," Watching Fishes is also a tribute to the inquisitive mind. While most divers are content to paddle blithely from one colorful underwater tableau to the next, the Wilsons pause and make mental notes, comparisons, educated guesses -- all of which they later check against the learned journals.

As described by the Wilsons, many forms of marine life exhibit the flagrant behavior that keeps Ripley's Believe It or Not flush with copy. Male octopi implant their sperm in females by "hand." Some parrotfishes protect themselves from nocturnal predators by secreting and coating themselves with mucus envelopes thought to disguise their scent. In self-defense some sea cucumbers go so far as to "eviscerate themselves, expelling from their posteriors not only their digestive tract and reproductive organs but parts of their respiratory systems as well." The point, apparently, is to distract the predator while the sea cucumber makes a getaway. Later it will regenerate the sacrificed organs.

Some fishes are sequential hermaphrodites, others simultaneous ones. The sequentials start out as members of one sex and change into the other. Simultaneous hermaphrodites "have the mature reproductive organs of both sexes and can reverse their sexual functions at will, performing either as a male or a female as the situation requires" -- something like the extra-terrestrial race in Ursula K. LeGuin's science-fiction novel The Left Hand of Darkness.

The book's most fascinating chapter sketches symbiotic relationships. Several kinds of crab take up residence amid sea anemones' poisonous tentacles. Themselves immune to the anemone's sting, the crabs rely on it to ward off predators. One kind of hermit crab obtains protection by kidnapping. The crab will pry an anemone loose from a rock, hoist it onto its shell, and travel about under the protection of a live-on bodyguard.

MORE COMMON are cleaning arrangements, in which small fish feed on parasites inhabiting larger ones. Sometimes this mutual dependence obliges cleaners to enter the open mouths of their clients. "A diver," the Wilsons note, "can often look into the mouths of large groupers and out through their extended gill openings to the marine landscape beyond while cleaners work them over." In the authors' own saltwater aquarium, two kinds of shrimp "will clean us as well as fishes. They will perform a thorough, almost systematic combing of the surfaces of our hands for parasitical matter; apparently our ectoparasites are as tasty to them as those gleaned from the sides of fishes."

There is a special chapter devoted to sharks, which, the Wilsons argue, are neither the "unpredictable savages" of popular mythology nor the benevolent beauties that -- overreacting to the stereotype -- some marine scientists have made them out to be. After reviewing the statistics, the Wilsons conclude that worldwide an average of five people a year die from shark attacks, "and these are rarely divers." Nonetheless, they offer advice for anyone confronting a shark: swim away smoothly.

The scrupulously honest authors repeatedly acknowledge the inchoate condition of piscatorial science. We know a great deal more about the behavior of ocean fish now than we did 20 years ago, when the Wilsons began diving, but the explanations for many common phenomena are still shaky. Take schooling -- the propensity of fish to swim in groups. It can be viewed as a method of saving energy: moving in "a kind of staggered checkerboard pattern," follower fish rely on the way-paving leaders to reduce the drag of water on the followers' bodies. Yet most fish evidently school to take advantage of safety in numbers and efficiency in predation -- which leaves the rationale for the prevalent staggered formation something of a mystery.

The loving labor that went into this book manifests itself on every page. Roberta Wilson's drawings are clean-lined and helpful; the photos taken by both husband and wife do justice to the fabled intensity of marine hues; and the text is well-keyed to both kinds of illustration. The reader who wonders, say, exactly what it means for one fish to "shadowstalk" with another need only consult Plate 17 to see how the trumpetfish swims just above -- and assumes the coloration of -- the less menacing foxface. For anyone wanting something extra out of his Caribbean vacation, Watching Fishes is an elegant subaqueous Baedeker.