BONAIRE, Netherlands Antilles - When the promoters of Caribbean tourism are scouting sites for their next travel poster, they sail right on past Bonaire.

The little speck in the Netherlands Antilles, just off the coast of Venezuela, cannot deliver the Caribbean's iconic images: vast beaches of powdered-sugar sand, lavish resorts with infinity pools, lush rain forests, pulsating nightclubs.

The island is a sparsely populated crescent of 112 square miles, and is nondescript at first glance. Until you look beneath the surface.

Of the water.

Ringing the island is an undersea world of astonishing complexity and beauty. At first plunge, the eyes of snorkelers and divers routinely grow wide behind their masks.

"The thing that's amazing about Bonaire, the water is always crystal-clear and the reef structure is so phenomenal," said Wilton Risenhoover, a dive master with the Eco Dive Center in Los Angeles and a frequent pilgrim to the island. "The reef structures are right there at the edge of the island - 50 feet out - and it descends 100 feet down to a sandy bottom. The water temperature is always 80 degrees. And if you get tired of one reef, there are 80 other ones you can drive to."

Indeed, the options are limitless. In 1979, the entire coastline of Bonaire was declared a marine park. That means that wherever you venture into the water, the environment is protected to a depth of nearly 200 feet - no fishing, no anchors, no spear guns, no collecting of coral, no walking on the coral. Everyone must pay a fee before entering the water ($10 for snorkelers, $25 for divers), which goes toward management of the park, including the patrols of enforcement rangers.

The island has 86 named snorkel/dive sites, including Ol' Blue, Sweet Dreams, Hilma Hooker and Alice in Wonderland, most of them concentrated along the calm, leeward western shore. In most cases, you don't even need to hire a boat to enjoy some superb underwater sightseeing.

Still, we climbed aboard the Sea Cow for a snorkel outing to Klein Bonaire, a tiny, uninhabited cay that shelters in a protective crook of the west coast.

It was an excursion on a Crystal cruise, and there were a number of older participants and novice swimmers. But the outing was suitable for all ability levels.

Whereas many snorkel tours tie up in a cove and have all the diving emanate from the boat, for this one we jumped into the water just off the coast of Klein Bonaire and swam to a narrow, sand-and-rock beach. Then everyone walked a good distance along the deserted shore, before swimming out through an opening in the reef.

A gentle current was working its way along the cay's edge, which created ideal conditions for a "drift snorkel" between Ebo's Reef and No Name Beach, all the way back to the boat. For many participants, this minimized the fatigue of a lot of swimming.

A bright sun was piercing the warm, aquamarine waters, illuminating an enchanting world. A school of blue tang skittered past. Some juvenile yellowtail damselfish were resplendent - purple, with neon-lavender specks.

But it was the marine life that wasn't swimming that was most alluring. Branching vase sponges. Yellow and brown tube sponges. Staghorn coral. Elkhorn coral. Sea rods, with delicate limbs waving as if in a breeze. This was an underwater botanical garden, and about as good as it gets.

At another stop, just off the main island at a site called Andrea II, there were more delights, including a parrotfish that was easily 3 feet in length and a green sea turtle that wasn't in the least mood for company.

Sea Cow Charters kept close watch on the 26 tour guests throughout. There was a crew of four on the trip, with three divers in the water and one member always staying on board.

As dive guide Marc Beenakkers took a break on the gunwale of the boat, he noted that Bonaire is increasingly finding its way onto cruise itineraries, which is introducing its wonders to a widening array of visitors.

The big ships, he said, "have only been coming here for the last couple of years. On days when the Crown Princess docks with a ship from another line, the population here (14,000) can swell by a third."

Bonaire always had a niche appeal for serious dive enthusiasts, and its tourism numbers reflected it - over the past several years, a daily average of fewer than 200 overnight guests. But today's cruise passenger in the Caribbean is no longer content with a bus ride to an old fort or an afternoon spent poking around in shops. Travelers of every stripe are keen for more communion with nature, even if it requires a breathing apparatus.

So, in the last month alone, Princess, Holland America, Celebrity, Crystal and even Cunard's storied Queen Mary 2 have all tied up here for a day. Bonaire welcomed nearly 92,000 day-trippers from cruises in the 2006-07 tourist season. For 2007-08, that is expected to climb to nearly 157,000.

It represents a tricky navigation for the island. Tourism is the main source of income here, having superseded salt production. But there's the implicit threat of the cruise invasions undermining Bonaire's simple, understated appeal.

"It is very important that Bonaire not grow massively on this front," said Ronella Croes, director of Tourism Corporation Bonaire, the island's official marketing arm. "One of the big advantages of Bonaire, it is very quiet and relaxed and not too developed. It's one of the things we want to maintain - the quietness and tranquillity. One of the impacts of having cruise passengers here, you feel it immediately, no?"

For the next several months, the only cruise ships visiting the island will be small ones - operated by the Windjammer and Majestic lines - carrying 100 to 300 passengers. Croes said the island is actively soliciting small-ship lines, rather than courting an armada of behemoths.

Because of this, Bonaire's largest town, Kralendijk, is the rare Caribbean port that has not yet been "cruise-ified." It is sleepy, unpretentious and utterly charming, with Dutch colonial architecture in a rainbow of tropical colors, as if the painting crews drew inspiration from the marine park itself. Farther inland, gleaming white homes with red-tile roofs are set in a grassy landscape.

At the waterfront, there is no press of entrepreneurs browbeating you to buy trinkets or take a taxi tour of the island, nor is there a ghetto of jewelry and souvenir stands. Shopkeepers sell dive gear, dive guidebooks and dive maps, and the ones we dealt with were friendly and low-key.

Most of the island's lodging is spread among small inns and vacation rentals. If you can do without ostentatious resorts or steel bands playing to dance parties on the beach, this might be the ideal get-away-from-it-all destination.

Maybe that's why the yacht of Microsoft co-founder Paul Allen - nearly the size of a Silversea cruise ship - was docked in the harbor on our visit.

"It's just so out of the way, it's lost," said Risenhoover, the L.A. diver.

He cited one of the most popular excursions on the island: a night dive beneath the Town Pier. Because of boat traffic, it has to be made with an instructor, but the payoff is perusing diverse organisms that have made the pilings their home.

"It was only about 20 feet (deep)," Risenhoover said. "You find these sponges in the middle of the night, and they light up with these brilliant purples and oranges and greens and pinks. You see an octopus. I remember the divers hovering there with their lights, looking like helicopters."

Just another moment of showtime on Bonaire.

IF YOU GO

GETTING THERE: Continental has nonstop flights from Houston to Bonaire (also from Newark, N.J.). The outbound Houston flight is a red-eye offered only on Friday nights. The next available return is a week later, on a Saturday morning. American has nonstops from Miami to Curacao, Bonaire's nearby neighbor. From there, you can fly the Dutch Antilles Express to Bonaire www.flydae.com). The cruise lines that will call at Bonaire through October are Windjammer www.windjammer.com) and Majestic www.majesticcruisesinc.com). Celebrity, Cunard, Holland America, Princess, Silversea and other lines usually schedule their visits during the Caribbean's winter high season.

EXCURSIONS: If you're going to be visiting on a cruise ship, this is one excursion you should reserve as soon as you book your cruise. The snorkel and dive boats are small, and these outings sell out quickly. A 2 1/2-hour snorkel trip to two sites is usually priced at about $60 on the large, midmarket lines. This includes equipment and the nature fee assessed by the Bonaire National Marine Park. If you don't want to go on an organized tour (whether you're staying on the island or visiting as a cruise passenger, the shore diving is outstanding), andwater taxis will run you over to Klein Bonaire, the island's tiny cay. You will, however, have to pay the nature fee - $10 for snorkeling, $25 for diving, good for one year - and get a tag to attach to your equipment. The permits are available everywhere - at dive shops, hotels, water taxis.

INFORMATION:The island's official tourism Web site is www.infobonaire.com. It offers extensive information about transportation to the island, vehicle rentals, lodging and snorkel operators, as well as dive shops, instruction and certification. Tourist office phone: (011-599) 717-8322.