Huddled together in a dripping cathedral of rainforest greenery, we flinch under the angry, ear-splitting screams of a dozen spider monkeys about fifty feet above our heads. Teeth bared in furious grins, the monkeys scream and race from branch to branch, raining leaves and monkey diatribes down on us. When they leap away to distant trees, I start breathing again.

The sudden silence begins to fill up with a steady murmuring, the white sound of the jungle, as a thousand birds, insects, frogs, and snakes resume their lives around us.

We are six tourists following a biologist guide along a spongy path, peering into the mass of green, watching for movement, listening for the rush of wings. Someone spies a coral-headed parrot. A scattering of tiny red frogs hops damply around the splayed base of a tree so tall its top is lost in a tangle of yellow orchids and Tarzan vines. Four feet long, with a jagged crest down its back, a fat, golden-orange iguana lazes on a fallen log. A hairy sloth sleeps rolled into a ball in the lacy canopy of a red-leafed almendro tree.

By noon, our list of sightings is long and we're drenched in sweat, splattered with mud, and ready for a cold beer and lunch. Slipping and sliding down the muddy trail, we burst out of the dark forest into a scene from a vacation brochure -- a palm-fringed curve of beach, the equatorial sun beating down on a warm sea, a white cruise ship glistening offshore.

Passengers on a "soft-adventure" ocean cruise, we've come to Costa Rica's Nicoya Peninsula to see the country's nature preserves and wildlife habitats In a week's time, we'll walk in the remote virgin forests of Corcovado National Park, take pictures of toucans in Manuel Antonio National Park, and explore pre-Columbian burial grounds at Caño Island. Costing not much more than a week of guided tours on the country's notorious washboard roads, the cruise is an easy way to see a lot.

The 185-foot, 100-passenger Pacific Explorer has a shallow draft, enabling her to anchor within dinghy distance of forest trails and beach accessible only from the water.

Like most of my fellow passengers, I chose the Explorer because I've sailed on the big ships, the royal ones -- the princesses, the queen, the monarchs of the seas, some carrying as many as 2,700 people. I've pigged out at mile-long midnight buffets, gambled in glittering seagoing casinos, and elbowed my way down gangplanks to crowded, touristy port cities.

Although she has creature comforts like hot showers, air-conditioning and 24-hour room service, the Explorer is not just another pretty ship. She is a small, comfortable craft for active travelers who want to be in Costa Rica's wild lands without dressing for dinner, or seeing power lines, people, or tour buses. Every day, a fleet of Zodiacs -- inflatable boats à la Jacques Cousteau -- takes us from the ship to a piece of Central America where few have gone before.

One morning, we hop off the Zodiacs onto Caño Island, following our guide into a dim primary forest of enormous cow trees and ancient cedars. Blindingly bright on the beach, the sun struggles to filter through the dense vegetation, weakly lighting our way on the forest floor. A pre-Columbian burial ground, Caño is dotted with looted grave sites. Scattered about are round stone balls four or five feet in diameter, weighting tons, brought here from the mainland centuries ago, no one knows how, to mark the graves. Poking around, we find half-buried fragments of stone bowls and figures. I kick up a heavy carpet of leaves and with them an oval stone, which turns out to be a smooth, carved head with a serene face, a suggestions of lips, a line for a nose, eye hollows, the beginning of a neck. Archeologists now, we huddle over our finds, snapping photos in a puddle of light. As a light mist begins to fall, we leave our artifacts at the site, returning to the beach on the narrow trail. The overwhelming green closes in behind us.

The afternoon is spent playing in the warm sea, kayaking, wind surfing, scuba diving and snorkeling. Zodiacs zip between the Explorer and the shore for those who want to sneak back to their cabins for a nap or a change of clothes.

Among the passengers are a silver-haired Californian in her 70s who teaches kayaking; a sprinkling of teenagers and their parents, honeymooners, a few singles, and married couples -- those who drink, play cards and stay on board; those who wear bikinis, wind surf and scuba dive.

Camera and binoculars dangling, I hang out with the bespectacled birdwatchers. One day we motor in small canopied boats on the Tempisque River, a labyrinthine waterway in a vast expanse of marshland and mangroves that is Palo Verde National Park. Thousands of migrating waterfowl rest here on their way south from North America. Pink clouds of roseate spoonbills swirl and squawk. Leggy, black-headed wood storks croak harshly at us as we put-put-put by their nesting grounds. Ospreys wear tuxedos, egrets their snowy gowns, shy herons are dressed in gray and blue. Black-bellied whistling ducks and pelicans glide low over the water.

Back on board, we compare waterfowl and fish sighting lists with those who spent the day diving. In the tropical dusk, I sip an umbrella-clad cocktail. The heads for a fiery-red horizon and I head for dinner.

In the glass-surrounded dining room, the atmosphere is informal. Tonight we're having grilled Pacific sea bass, and steaks from Argentina. Traditional Costa Rican gallo pinto -- beans and rice -- is a staple at every meal, along with platters of fresh tropical fruits. Not be outdone by the luxury liners, the chef creates a nightly extravaganza of ice sculptures and flaming desserts.

On deck, the bar stays open late, and in the lounge, videos recap the day's adventures. Our conversations are drowned out for a few moments while the anchor chain clanks up. The engines rumble-rumble into life, then subside to a steady throb. In a midnight blue sea, dolphins ride the bow wake. Skipping along for miles, flying fish sprinkle moon dust on the waves. We drift off to our cabins as the Explorer sails quietly to tomorrow's anchorage.

We're off to Caletas, a privately owned tract of jungle frequented only by Explorer passengers. Joining us is Antonio, the father of the family living here in a thatched-roofed, metal-sided house. He leads us beside a stream into the rainforest. Mist hangs in a high umbrella of dripping leaves. The forest tunes up -- chirps, grunts, squeaks, howler monkeys moaning, toucans cracking nuts, blue-black manakins yodeling what sounds like "to-le-do, to-le-do". Antonio whispers, pointing to a violaceous trogan, a dazzling, long-tailed green bird in a giant cedar. Crowding the 200-foot trunk and the branches of the tree are philodendrons with leaves the size of Volkswagens, and hundreds more plants, called epiphytes, which are plants that rely on the cedar for support, but not nutrients. These are ferns, mosses, waxy-pink bromeliads, swaying vines, and waterfalls of red orchids, their tendrils and roots inhaling food and water from a fragrant, nearly liquid ocean of air.

Antonio tells us about the female jaguar and her cubs that he saw fishing in the stream a few days ago. My senses now on full alert, I jump when a tribe of monkey suddenly comes crashing along in the branches.

Later, we lounge in Antonio's beachfront yard, drinking rum out of coconut shells before lunch and an afternoon of snorkeling and tidepool wading.

The cruises ends with a moonlit beach party at Drake Bay, the Explorer a comforting glow offshore. Crew and passengers dance around a bonfire, sparks floating into a inky sky shot with stars. One of the biologist guides, an Ecuadorian from the Galapagos Islands, plays his guitar and sings. A kitchen bands arrives by Zodiac, the chef and his helpers dressed in coconut bras and apron skirts, drumming a Latin beat on pots and pans.

The jungle behind us is noisy, too, moving with life. Monkeys chatters, poison dart frogs squeak, something rustles in the dark. A fine rain begins to fall. We're saturated with Costa Rica.