Great Barrier Reef. Taking a plunge into a teeming world of Picasso shapes and Tiffany colors on Australia's

AT a depth of 75 feet, drifting between the shelves of a coral canyon, life on the Great Barrier Reef begins to knock on the door of one's quiet senses. Jacques Cousteau referred to the watery depths as ``the silent world.'' Indeed, only the rhythmic muffled purr of bubbles flowing from our regulators interrupts the stillness.

Silent, yes, but colors and shapes unimaginable just a few short minutes ago begin to scream for attention. Fish so bizarre, in Picasso shapes and Tiffany brilliance, dart like wingless finches between branches of coral. Here on the reef, it is said, is more life per square inch than anywhere else on earth.

Even without a single fish, the reef would lose little splendor. The soft and hard corals, made up of countless polyps, are endlessly fascinating in color, variety, shape, and size. It is these tiny, delicate animals growing from the skeletons of their ancestors that make up the reef and hold back the force of the waves.

Blizzards of silvery fish, pencil-thin, flash toward us in perfect precision, then, taking a cue from who-knows-where, change direction in a fraction of a second, catch the light, and vanish into the deep.

Orange-and-white clown fish sway in a sort of seductive hula, teasing unsuspecting prey into the poisonous, soft, pink-tipped tentacles of a sea anemone. In this symbiotic relationship, the clowns have the last laugh. A mucus coating makes them immune to the anemone's deadly touch. Others are not immune.

And to think, just minutes before, the four of us were floundering about the deck in swim fins, staggering under the weight of 60-pound diving belts, struggling to keep our balance aboard the bobbing launch as assistant divemaster Terry Kennedy patiently answered our many anxious questions.

``No,'' he said, ``you can't take any coral or anything, dead or alive, from the reef. It's a national park, remember?'' And, ``Yes, we will take another 45-minute dive in a different spot.'' And, ``Yes, you have to buddy-up and stay together.'' And, ``No, you don't have to wear gloves, but some of the coral is sharp and dangerous, so you'd better.''

Then, with a nervous laugh, someone asked The Big One - ``What about sharks? Any chance of seeing a shark?''

``Oh, maybe if we're lucky,'' Mr. Kennedy said offhandedly, without so much as looking up.

Not quite the direct, firm negative we were looking for.

Moments later we plunged into the warm waters and drifted slowly down into the azure depths.

As we made our way through a coral tunnel, Kennedy stopped to coax a six-foot moray eel from its lair with tidbits of fish. ``Monty'' -- as Kennedy named him - was usually there, he explained. Monty would lunge out and grab a bit of fish while Kennedy tried to pet him on the head. Being in no mood to be petted that day, Monty slipped farther back to his dark lair.

Over here, Kennedy motioned, as we moved in on a lion-fish fanning its feathery orange-and-white pectoral fins. We knew enough not to get too close. Each exquisite fin houses a poisonous quill. All of us were familiar with this exotic beauty so coveted by home marine aquarists throughout the world.

Slipping deeper down to a sun-dappled coral garden, we spotted a lone three-foot giant clam lodged in the sand. A thick mantle, velvety and purple, covered its ``jaws,'' which slowly closed as we tapped gently on one bivalve. There has never been a recorded case of anyone's being caught by these ``man-eaters,'' we were told. They simply react too slowly to threaten divers. These gentle giants live for generations, growing to over 400 pounds, and are now protected on the reef.

We hung suspended in wide-eyed amazement while a group of fish waited patiently as a thin, lithe cleaner wrasse swam in and out, from fish to fish, entering each mouth, cleaning lips, mouth, and gills, and moved to the next. Even- larger fish that could eat a wrasse as an appetizer would turn and roll about with mouth open wide as the little cleaner went about its business.

The figures here are staggering: more than 1,500 identified species of fish and 350 species of coral on this 116,000-square-mile, 1,250-mile-long, 2 million-year-old reef. The Great Barrier Reef, we were told, is larger than all the man-made objects in the world put together! We swam spellbound in what amounted to little more than a drop of this underwater wilderness.

Kennedy dipped down to pick up a slippery, bulbous black sea cucumber a foot long and tossed it as we grouped for a slow-motion game of touch football - only to be interrupted by a hawksbill turtle that came by, blinked, turned, and left looking quite bored. We dropped our slippery sea creature and gave chase.

Five gold-and-blue angelfish drifted by as we swam between a stand of honeycomb and fan coral. One male and his four mates. If a male is lost, we learned, the dominant female quickly changes sex and takes over the harem.

Parrot fish in rainbow hues chipped away at the coral reef, spitting out tiny bits that add to the countless grains of coral sand.

Kennedy almost lost us as we followed him thorough a school of hundreds of midnight-blue surgeon fish, then out into a coral garden and down to a sun-dappled clearing.

Click, click, click. He tapped on his watch to attract our attention and pointed toward a large gray shadowy figure lurking ahead. A shark, as surprised as we were, if not more frightened, cut a few sweeps of its scythelike tail and disappeared into the sea as quickly as it had appeared, leaving us with hearts throbbing in our throats.

Seconds became half an eternity.

No, the most frightening thing is not seeing a shark, I discovered. It is seeing one -- and then not seeing it.

Kennedy's face mask hid a broad smile, but his squinty eyes, nodding head, and thumbs up signaled his delight as the four of us looked nervously about for the intruder, or worse, some of his friends. Fortunately, we saw the shark on the last minutes of our second 45-minute dive.

Up, Kennedy pointed after checking his watch, and we slowly made our ascent. ``Did you see it? Did you see it? Did you see it?'' we sputtered as we clamored like trained seals aboard the launch. ``It must have been at least eight feet long!'' ``Oh, at least. Maybe 12,'' we argued.

``Four feet,'' Kennedy said coolly.

``Four feet? Come on!''

``A four-foot reef shark. Quite harmless,'' Kennedy said. ``Water magnifies everything 25 percent.''

``Well I'm not telling anyone it was less than an 18-foot great white,'' one of the guys laughed as we peeled off our wet suits. Good story, we all agreed as we headed back to the catamaran.

Exhausted, excited, relieved, and wildly embellishing on how we had survived the jaws of a school of 18-foot great white sharks. Would 20 feet be overstating too much, we wondered? As for the beauty, power, and majesty of the reef, it needed no such elaboration. Practical information.

The new MV Quicksilver II offers the only daily cruise to the actual outer Great Barrier Reef. This high-speed catamaran is capable of carrying hundreds of people at a speed of 26 knots from Port Douglas to the reef, where it moors for 3 hours. The 90-minute trip out includes lectures on the reef and video and slide presentations by a marine biologist. Use of diving and snorkeling equipment is reviewed. You don't have to scuba-dive. In fact, only four of the more than a hundred aboard were certified divers. To scuba-dive, you must have your certified diving card with you.

Most people are content to snorkel off the side of the platform in 35-foot water. Others view the reef from the glassed-in side of a floating platform, or buzz around in the Subsee Explorer, a kind of submarine. You can view this watery wilderness without even getting your big toe wet. Any way you choose, the show is unforgettable, the cast is outstanding, and the sets breathtaking.

A sumptuous buffet with fresh prawns and endless salads is included in the fee, and underwater cameras are available to rent.

Contact Low Island Cruises, Point Douglas, Queensland 4871, Australia. Telephone (070) 98 5373. Telex AA48969. Port Douglas caters to divers. Qualified instructors have shops throughout the area. You can become a licensed diver in a few days.

If you want to do some homework before you go, a marine biologist on board recommended the ``Reader's Digest Book of The Great Barrier Reef.''

Lodging is available and rates are moderate. I can think of no better place, however, than Silky Oaks Colonial Lodge, just 83 kilometers (50 miles) from Cairns International Airport, or 27 km (half-hour drive) from Port Douglas.

Silky Oaks is a chance to see a totally different, equally fascinating side of this tropical area. Here you can live among the flora and fauna found only in this part of the world.

Moss and Theresa Hunt have cleared acres of rain forest and set up very comfortable, stilted Queensland-style cabins overlooking Mossman River Gorge. There, in the morning you can seek out a platypus searching for breakfast in a billabong, or swim among the trout while blue-crested kingfishers dive overhead. You may hand-feed bread to the wild turtles along the bank, canoe on the river, or hike through the surrounding rain forest.

Or just sit on your individual balcony and listen to the water and watch green birdwing and blue Ulysses butterflies flit from blossom to bud.

As Moss describes it, Silky Oaks is ``living in a park - with plumbing.''

One-, two-, or three-day safaris in an air-conditioned four-wheel-drive are also available. And be sure to visit the local butterfly farm. Moss will be happy take you there and be your guide along the way. He knows the names of everything that flies, crawls, or grows in the area.

Rates at Silky Oaks are moderate and include three meals a day. Both Moss and Theresa are Cordon Bleu-trained cooks, to boot.