It's 9 o'clock on a moonlit evening, and in the Marco Polo Lounge of our cruise ship, the Captain's Gala is in progress. Outside, on the Promenade Deck, I am prostrate in a deck chair. My chiffon evening gown -- my Captain's Gala gown -- is crushed under a scratchy woolen blanket.

I don't give a hoot.

I am seasick.

When I raise my heavy eyelids, I see a row of 11 other gowned women. They don't give a hoot either.

As we offer up our limp arms to the ship's doctor for anti-nausea shots, he explains that the ship is both pitching and rolling, a phenomenon known picturesquely as "corkscrewing." This stomach-churning motion results when winds and currents clash, as we leave behind the island-sheltered Caribbean waters and enter the Atlantic Ocean en route to Venezuela.

The ship's stabilizers, we are told, are ineffective against corkscrewing.

Great.

This shot had better work, I think darkly. In 27 hours my husband and I will celebrate our 25th wedding anniversary. This cruise with its fascinating side trips -- flying over Angel Falls in Venezuela and exploring a remote jungle camp -- is our long-awaited anniversary trip. Now I'm wondering whether I'll be able to revive enough to enjoy it.

I moan piteously, but only the ocean hears me; my husband is in the lounge, happily scarfing up champagne and hot hors d'oeuvres at the gala.

Ten hours later, I awaken to find that the ship has finally reached the "sweet waters" at the delta of the Orinoco River. What a change from the day before, when we had skimmed through the sunlit, sparkling Caribbean as we island-hopped through the lovely Grenadines. Now, the water lies flat, muddy brown and sluggish looking. The sky echoes the flatness in shades of dull gray. Overnight, we have glided into a different world.

Our destination, 180 miles upriver, is the industrial center of Ciudad Guayana, the stepping-off point for our air excursion to Camp Canaima in the jungles to the south of the city.

Our guidebooks extol Camp Canaima, a secluded retreat on a palm-fringed lagoon fed by seven waterfalls and surrounded by orchid-filled jungle -- a corner of the Garden of Eden. En route to this paradise, we are to fly over Angel Falls, the highest falls in the world, 15 times higher than Niagara. The view of the falls is the glory of Canaima National Park -- if the weather permits.

The day passes peacefully. The setting is spectacular, although I am a bit disappointed at first; enchanted by the language of the guidebooks -- "exotic Orinoco River ... sprawling jungles ... spectacular mountains and unusual wildlife" -- I had imagined an African Queen setting of overgrown riverbanks and chilling jungle sounds. Common sense should have told me that the mighty Orinoco would also be broad -- so broad that field glasses are needed much of the time to see clearly the scattering of Indian dwellings on the riverbanks.

Nevertheless, as we steam silently through the broad, brown water, I have the adventurous sense of truly being off the beaten path. We had selected the small (250-passenger) Ocean Islander because larger cruise ships are unable to navigate the Orinoco. (To accommodate large merchant vessels, dredges work year-round vacuuming up silt to keep the channels clear, and even then the ships must wait at anchor to move with the tides.)

The other passengers also seem to sense the remoteness, for conversation is pensive and subdued as we stare out at the jungle surrounding us.

Occasionally there is a flurry of excitement when Warao Indians paddle their dugout canoes to the ship to pick up the plastic bags of clothing, cookies and apples that our crew tosses overboard. Apples are especially welcome because they're rare and expensive -- the equivalent of $4 apiece at Indian trading posts.

My sense of otherworldliness increases as we go deeper into the heart of Venezuela and see no more than a dozen scattered "settlements" of three or four Warao homes built on stilts made from palm trees. The Waraos use the tall, strong and willowy palms to thatch their roofs and to make a syrupy brew. They also eat the grubs -- considered a delicacy -- that live in the trees.

A water people, the Waraos make their living on the river in large dugout canoes, which they skillfully carve from a single trunk of the cedro tree. They barter for fuel and "store-bought" needs at scattered trading posts in the jungle.

At dusk, the cruise director alerts us to watch for parakeets, macaws and egrets heading for the trees to roost for the night. As if on cue, suddenly a flock of egrets swoops down from out of nowhere, gliding and settling with a gentle flurry on a wide-branching tree. I gasp at the sudden beauty, for, in the fading light, each bird looks like a giant white rose on a lushly flowering bush. They are so thick in the branches that the tree itself becomes a huge, moon-white blossom glowing on the darkening shore.

The beauty is cut short when a swarm of mosquitoes as thick as the egrets attacks the ship and drives us, flapping our arms, into the passageways.

In the morning, under skies thick with white clouds, a bus takes us to the airport of Ciudad Guayana, known as The City of the Confluence of Rivers. Here the yellow-brown Orinoco and the dark Caroni River meet and swirl together dramatically, like ribbons of chocolate and peanut butter fudge.

But our attention, once we are airborne in a 727 jet, is riveted on the thick cloud cover. Will we see Angel Falls? It's located in a region of dense jungle where ancient, cloud-wrapped, flat-topped mesas tower over grassy savannas. The cataract leaps 3,212 feet from Devil's Mountain down into Devil's Canyon.

The falls take their name from American adventurer Jimmy Angel, who, in 1937, with his wife, Mary, and a geologist friend, mistook the shimmer of the falls for the glint of gold and tried to land on the mesa. The plane crashed, but, fortunately, the explorers met some friendly Indians who took one look at Mary's long blond hair, proclaimed her a goddess and lay a sack of diamonds at her feet before leading the threesome out of the jungle.

Our cheery pilot announces that if cloud conditions permit, we'll descend from our flying altitude of 12,000 feet into Devil's Canyon for a flyby of the falls.

At the canyon the clouds seem thicker than ever, so my heart jumps when the pilot casually says, "It's not the best -- the clouds are very low -- but we're going to try to fly into the canyon."

"Try?" The word is whispered shakily through the cabin.

Try he does. Determined to give us our money's worth, he takes that little jet down into the canyon, popping in and out of clouds and dipping his wings smartly to improve our view.

And what a view -- when we can see it. The windows of a 727 are only slightly larger than a person's face; moreover, most of the time we are cocooned in clouds.

But, then, suddenly, there's a break, and for a breathtaking moment the sheer rock walls of the canyon loom startlingly close -- just outside the window. In the next instant we are cloud-bound again.

"Angel Falls coming up on the right side!" the pilot announces.

A moment later I glimpse it. Well, the lowest third of it -- a thin, pewter-colored streak against the dark rock wall. And then it's gone, socked in by clouds.

Our pilot, as daring and skilled as he is cheerful and determined, banks, turns and zooms us back through the canyon once more so the left side can have a shot at the falls.

But to no avail. The clouds are thickening, and we breathe easier when we climb out of them. Despite my disappointment, the uniqueness of the flight, with its hint of danger and the spectacular, sheer rock cliffs looming so close to us, greatly makes up for missing the falls.

The previous week, the pilot informs us, the view had been magnificent.

Twenty minutes later, when we touch down at a small, forlorn airstrip in the savanna at Camp Canaima, I look around warily.

This is the Garden of Eden? I'd imagined a personal paradise -- sun-kissed island, crystalline waters and white sand beaches.

The jungle setting, especially under cloudy skies, casts an otherworldly, primeval spell. The campground itself is rustic but tidy -- 60 unadorned, thatch-roofed cabins shaded by heavy tropical foliage.

But the lagoon! Here, truly, is the Garden of Eden -- pure, crystal-clear, blood-red water and fine, pink-tinged sand. Across the lagoon, at Hacha Falls, the Carrao River thunders over seven huge cataracts. Clouds of spray and mist veil the forested cliffs between the rushing waters. Beyond the falls, a flat-topped mesa soars like a mighty fortress, straight up into low drifting clouds. It looks like the beginning of time.

Most of our group chooses to go on excursions with local guides -- a two-mile jungle walk or a jeep ride over the savanna. Both excursions earn good, but not enthusiastic, reviews. (Total jungle wildlife seen en route: one green lizard.)

After a short, motorized canoe ride that takes us exhilaratingly close to the falls, my husband and I set off to explore -- and to swim in the lagoon, which gets its striking color from tree roots and minerals in the soil. Close to shore, the water is a light tea color, which deepens as the water does, to amber, then to cream sherry and finally to blood-red -- but always incredibly crystal clear. And potable. Not far from us, half hidden in the reeds, an Indian woman scrubs clothes, while two naked babies play in the shallows.

In the nearby Indian village three bright-eyed girls smile and pose shyly for us in front of their one-room schoolhouse. In their neat school uniforms, they are a cheerful contrast to the dreariness of the rubble-filled yards where chickens and mangy dogs scratch in the dirt.

Our final stop is a trading post straight out of a Somerset Maugham story -- a dim, doorless building of dirty white stucco, topped with rusty sheets of corrugated tin. Its jumble of wares include stacks of battered crates, T-shirts strung from the ceiling, purple-sequined earrings, dried gourds, dented gasoline tins, palm frond hats, dusty packs of crackers, roughly hand-woven baskets and hand-carved blowguns -- real ones! In the midst of the clutter a bone-thin old crone swings in a hammock, while a huge parrot skitters about underfoot. A sign says, "We accept Mastercard and Visa."

On the flight back, our pilot/tour guide points out a diamond mine below us. I think wistfully of Jimmy and Mary Angel. They saw Angel Falls up close and personal and got a sack of diamonds for a souvenir.

And me? No Angel Falls. No diamonds.

But I do have a really neat blowgun.

WAYS & MEANS

GETTING THERE: The Ocean Islander is no longer sailing the itinerary we took (in fact, it's been sold and renamed the Royal Star). However, other cruise lines offer excursions that include the Orinoco River:

Clipper Cruise Line's Yorktown Clipper, a 138-passenger ship built in 1988, will make 11-day trips between Curacao and Trinidad, departing Dec. 9, 19 and 29, 1990, and Jan. 8, 18 and 28, 1991. Ports of call are Bonaire, the Los Roques archipelago, the Margarita Islands, Tobago, the Orinoco River (including a flight over Angel Falls, weather permitting), Curiapo and Ciudad Guayana. Fares start at $2,400. Information: 7711 Bonhomme Ave., St. Louis, Mo. 63105, (314) 727-2929 or 1-800-325-0010.

The Society Adventurer, a brand-new 160-passenger ship of Society Expeditions Cruises Inc., will sail on a 17-day expedition beginning Oct. 5, 1991, from Barbados to Trinidad, including the Orinoco River.. Costs start at $5,950. Information: 3131 Elliot Ave., Suite 700, Seattle, Wash. 98121, (206) 285-9400 or 1-800-426-7794.

Salen Linblad Cruising's new 164-passenger Frontier Spirit begins a similar 17-day itinerary Oct. 3, 1991, for $5,685 and up. Information: 133 E. 55th St., New York, N.Y. 10022, (212) 751-2300 or 1-800-223-5688. WHAT TO TAKE: Pack your own seasickness-prevention medication -- costs on board for an anti-nausea shot or a behind-the-ear patch can be expensive. Pack field glasses and an extra pair of sneakers for drenching jungle walks. Note: Inoculation for yellow fever may be recommended for this cruise. RECOMMENDED READING: "Jungle Journey to the World's Highest Waterfall," in the November 1949 National Geographic magazine, by explorer-photojournalist Ruth Robertson, makes great background reading.

Barbara Morris is a freelance writer in Alexandria.