SOME YEARS BACK, the New York Yankees, in an effort to fill their stadium, came up with a terrific catchphrase: "At any moment, a great moment." I recall the slogan when I'm about 60 feet beneath the Celebes Sea, diving along the precipitous coral wall that surrounds Indonesia's Bunaken Island. Vitro Tumpia, a mocha-skinned scuba guide from Froggies Divers, raps his tank with the hilt of his knife—a quick, dull thunk that signals such a moment has arrived.

We make eye contact, and he points. Just below us, a trio of blacktip reef sharks slips by, their vacant eyes and efficient jaws stirring a limbic dread. Their passage alerts a hawksbill turtle; she emerges from her lair and fades into the distance like a slow-moving UFO. In just 40 minutes, we've encountered scores of fantastic creatures: a tiny orangutan crab covered in crimson mohair, iridescent mandarin fish hiding among the red-tipped branches of a fire coral, and a family of ribbon eels, cartoon-character snakes with electric-blue bodies streaked with neon yellow.

All very enchanting, to be sure. What I'm really looking for, though, is one of Bunaken's most elusive creatures: the ghost pipefish, a surreal cousin of the seahorse that's as weird as anything out of Dr. Seuss's imagination. Two days ago, I'd never even heard of these animals, but my short stay at the Froggies bungalows, with its expert and obsessive clientele, has already had an impact. My days of blithe diving are over; now I'm keeping score.

Off the northern tip of Indonesia's Sulawesi lies Bunaken, a tiny island where the marine ecologies of Australia, Malaysia, and New Guinea converge. Bunaken, Indonesia's most impressive national marine park, is one of the country's success stories. It covers nearly a quarter-million acres, 97 percent of which is underwater. The modest entrance fee buys you admission for a year. (In contrast to many Indonesian national parks, which allegedly enrich Javanese bureaucrats, every rupiah of Bunaken's fee is invested in reef and island conservation.)

On a globe, this part of the earth looks as remote as Betelgeuse. When you get there, of course, it's a different story. Divers from all over the world consider North Sulawesi province an undersea mecca—a place to commune with rare varieties of nudibranchs, octopuses, and exotic fish. For many of these adventurers, Froggies, Bunaken's best dive resort, is a home away from home.

Froggies is a funky, palm-shaded spot with eight bungalows, bucket baths, and plentiful seafood buffets. Geckos patrol the ceilings. Small waves lick the shore with a jazz-brush cadence. Formal entertainment is scarce, but one of the dive guides usually has a guitar in hand.

The queen of Froggies is Christiane Muller, a spry, 67-year-old, sun-ripened French-Italian woman with precise English and a dry sense of humor. Before her first dives (in 1988, at age 50), she'd had a variety of careers: DNA researcher, translator (she speaks six languages), and ethnic-music producer. It was on a trip to the Caribbean for music research that her youngest son invited her to go scuba diving.

"I told him I couldn't possibly," Muller laughs. "And he said, 'Oh? I dare you.' " She grinds out her clove kretek in a glass ashtray. "The moment I touched the water, I had this feeling: I'm home. Now I get up every morning wondering, What will happen today? What will I see?"

Muller began her dive-instructor career on North Sulawesi in 1993. Two years later, she moved to Bunaken and hung the Froggies banner. Today, the shop is a magnet for serious divers, some even returning for a second or third visit. They're a focused bunch: No package tourists here; this is a three-week, three-dives-a-day crowd. Most of the guests have logged hundreds of dives in their careers; some have cleared a thousand.

After dinner, the ten divers—two Belgians, two Germans, three Italians, a French couple, and me, the lone American—sit around the table, trying to remember the names of all the Seven Dwarfs. A cross-cultural puzzler, but we finally succeed, with the unobtrusive Happy as the last holdout.

Then comes the real fun: poring through Froggies' zoology library, identifying the day's critters. Maybe I'm deficient, but I've never cataloged the creatures I meet on dives. On Bunaken, though, I'm surrounded by aqua-twitchers—people who compile lists of every Amphineura and beryciform they've seen.

Oppressed by their jargon, I wander off to Muller's bungalow. I express my impatience with this fetish for Latin nomenclature, but she simply shrugs. "When I started diving, I had the same feeling: I'll never be able to remember all these names." She looks at me slyly. "But you develop an interest."

Of course, she's right. After a few dinners with this lot, a photograph in the book Seahorses, Pipefishes and their Relatives gets under my skin. I find myself fascinated by a creature called the ghost pipefish.

Ghost pipefish (Solenostomus paradoxus) are like marine hummingbirds: delicate, multicolored creatures that hover around the nutrient-catching petals of feathery echinoderms. They're elegant and adorable, and—it becomes evident—everyone's seen them but me. The diving scorecard is out. I must make my mark.