Thirty feet below the surface, the color red disappears. Your lips look gray; peel them back and your gums are blue.

As you descend farther, you lose orange, then yellow, green and so on through the spectrum, until presumably even the deep violets look black.

But we are not going that deep this afternoon. We'll bottom out in Snapper Valley, 80 feet below the surface, where a green sea turtle lumbering behind a giant tureen of coral appears painted in delicate sepia against a pale aqua backdrop.

Please do not be fooled: I am no undersea jock. I'm sitting dry in an air-conditioned, 48-passenger Atlantis submarine, peering into the water through a porthole the size of a trash can lid. This is underwater exploration for the Discovery Channel set, reef diving performed from a butt-form plastic seat. The first red thing I saw lose its color was the soda stain on my son Jordan's T-shirt.

Our dive takes place about five miles offshore from St. Thomas, in the U.S. Virgin Islands. As one of the most visited cruise ship ports in the Caribbean, the island is full of fun, if not quite authentic, attractions like the Atlantis submarine. Any water activity that might appeal to a tenderful of cruise ship evacuees -- fishing, jet-skiing, sailboarding, water skiing, snorkeling and scuba diving -- is available. There were so many parasailors in the air one day that I wondered if the folks in the tower at Cyril E. King Airport should be keeping an eye on them.

I don't mean to sound like one of those snobs who look down on islands that have ATMs and fluoridated drinking water. You can work hard and find some remote pockets in St. Thomas -- the proverbial deserted crescents of white sand backed by verdant mountains rising from the sea.

But mostly you'll find decent resorts and condos on handsome beaches, along with a bewildering number of jewelry and liquor stores, a tiny coterie of Danish colonial buildings, a small theme park, an upmarket golf course and development that ranges from Euro-chic to still-not-rebuilt-from-the-last-hurricane cinder block.

You'll also find a tram leading to a magnificent mountaintop view of Charlotte Amalie. From the bar and restaurant at the top, you enjoy an expansive vista of the coast, the city and the harbor -- and of the cruise ships that provide customers for all of it.

Ah, the cruise ships. Don't even think of going to St. Thomas if you don't like the idea of sharing space with -- or having to navigate your way around -- the thousands of people who visit the island via Carnival, Royal Caribbean, Disney and other cruise lines. In 2003, 817 cruise ships brought more than 1.6 million people to the island for a visit.When the ships drop their human loads, the tiny city of Charlotte Amalie clogs with shoppers, some very polite cabbies form a gantlet by the pier, and buses full of day-trippers scatter among the historic sites, beaches and natural features that appear on guidebooks' must-do lists.

But there are days when no ships are in port and (if the attractions are not closed as a result) you have the run of the place. Even on days when the ships cast their shadows on the capital, you can do some work-arounds. The legendarily beauteous Magens Bay beach is deserted every morning until about 10 a.m. The Mahogany Run Golf Course, a Fazio design with a cluster of cliffside holes called the Devil's Triangle, has wide-open tee times most late afternoons.

Because of the "U.S." part of the U.S. Virgin Islands, the island has a familiar feel. This is either good or bad, depending on one's viewpoint -- and depending on how "ugly" the Americans on either side of a transaction happen to be. Everyone speaks English, traffic is messy, parking can be a chore, and Burger Kings and KFCs are always within reach. There are several unsettling billboards warning locals about the big fines for owning handguns, and street crime is a well-reported problem, especially at night in the menacing alleys just a few blocks from the jewelry stores on Main Street.

But there's still plenty to recommend St. Thomas, including the Atlantis sub, where my two boys and I and about 30 others enjoyed a fine performance by a swarm of curious yellowtail snapper, a small band of indifferent Caribbean reef sharks, two sea turtles, a flutter of stingrays, and waggly schools of blue chromis, French grunts, striped damselfish, beaky parrotfish and groupers the size of kindergartners trailing droopy Rasta mustaches. We saw brain, pillar and fan corals. A handy laminated card chained near the porthole helped identify them all.

Like so many travelers, my family and I like to investigate our vacation destinations beneath the surface. In this case, we wound up taking that literally. We explored the waters around St. Thomas from, first, the surface itself (via kayak). Then we checked things out from just below (snorkeling), and from about 25 feet down (more about that later) and finally from the Atlantis sub. It was a multi-layered view of the water, a kind of sea-life parfait.

The high point was not at the bottom.

Enter the Lagoon

We started our island exploration at the Mangrove Lagoon, which sounds like a water park ride but is one of the most ecologically important features on St. Thomas. Guided by the sort of cheerfully earnest folks who spend their days doing what others pay for, about a dozen of us paddled flat-bottom kayaks around the sanctuary.

We learned all kinds of stuff: that the mangrove trees sip seawater and convert it, via osmosis, into fresh water they can drink; that they form a natural barrier from the pounding waves of the Caribbean and a filter between the developed island and natural coast; and that, most significantly, they offer shelter to juvenile fish. The small fry grow safely in the shallows of the lagoon until they can fend off predators.

We got to fend off some creatures ourselves when we beached our craft and put on snorkeling gear. We had to keep our life jackets on so we didn't scrape the life-teeming bottom with our (unflippered) feet. "If you even graze the coral, it dies," a big guide with a wide hat said repeatedly.

So warned, we mostly just bobbed face-down on the surface, spread-eagled, to view the bounty below, daintily wrist-paddling between areas. We did indeed see junior versions of the grown-up fish we saw elsewhere darting around in nervous miniature schools. Often we hovered just a foot or two above some upside-down jellyfish throbbing lightly on the sandy bottom.

The guides also explained -- as did several naturalists we met during our trip -- that the white sand of St. Thomas's beaches is produced almost entirely by parrotfish. It appears the creatures nibble the coral and excrete the white crystals in quantities vast enough to line the island's shore.

This made for some interesting thoughts when, determined not to unsettle the delicate web of sea life, I snorkeled back to shore nearly all the way in rather than standing up when the water was knee-deep.

I gently beached myself, chin-down, in parrotfish poop.

The Mask

People will tell you that St. John, the more rustic Virgin Island neighbor of St. Thomas, is the place to go for snorkeling.

They are right. Still, we liked the snorkeling on St. Thomas just fine. In fact, we chose our hotel -- the mid-priced, decently appointed Sapphire Beach Resort -- for its location. In addition to two pools, three restaurants and a full water sports concession, the place has several good coral reefs right offshore. This meant we had access to both walk-up snorkeling and swim-up cocktail bars at the very same resort.

Any reef so close to people slathered with SPF 15 and carrying plastic cups of margaritas is not going to offer the pristine coral formations that make for the best snorkeling. But we saw plenty of underwater life anytime we liked. Those ubiquitous yellowtail snappers were there, too, along with blue chromis and some bone-white cousins muttering along the bottom, sometimes in water no deeper than your armpits.

Since I'm not a great swimmer I'm no great snorkeler, so I spent a lot of time in the shade of the scraggly beachfront trees, nursing minor coral burns and an iced Scotch. But our teenage boys spent hours out there just below the surface of the Caribbean. One afternoon Jordan showed off a starfish, an orange crab and a sea snail and returned them to their rightful places without incident.

On other vacations, snorkeling was a planned excursion -- you made reservations, hired transportation, plopped in the water, enjoyed yourself, then returned home. Sapphire Beach was more like a drop-in, all-you-can-snorkel buffet, serving fresh sea life whenever you're in the mood. Once I went out to the reefs when it was nearly dark, just because I could. I didn't see much, but that wasn't the point.

Walk This Way

Our best subterranean adventure came on a reef walk called Sea Trek, where we were escorted along a nature trail about 20 feet below the surface. It's located at the Coral World Ocean Park, a sort of shabby U.S.V.I. version of Sea World.

The underwater stroll is possible thanks to a large white Buzz Lightyear helmet that sits on your shoulders. Like a cup held mouth-down underwater, the helmet holds a cavity full of air. An oxygen hose running from a pump on the surface to the rear of your helmet provides pressurized fresh air that eventually escapes via small holes in the back.

This takes a few minutes to get used to, standing seven meters below the surface, your head surrounded by a bubble of oxygen, wearing just a swimsuit, gloves and underwater booties. Your breath sounds like the guy in "2001" when he's trying to disarm HAL.

But just like the guides topside promised, you can breathe just fine in that get-up, and even when you tilt your head the water doesn't rush in. Once you quit worrying, Sea Trek may be the best way to experience the Caribbean that doesn't require certification or a propeller. My son and I spent nearly half an hour loping like bare-chested astronauts along the path, which is lined by a fat metal chain. The eye-level reefs display the usual assortment of tiny speedy fish, exotic fan corals, and various organic attachments like anenomes and little olive-colored pennants that drift in the currents. We saw a trumpetfish, an eight-inch, eel-like thing that shimmies at a 45-degree angle, two yards away. Elsewhere we'd seen trumpetfish recoil fiercely into a slingshot and strike prey, but here it gave us no alarm.

When a sea spider drifted by, a guide "captured" it by surrounding it with his hands and pushing it toward us. Imagine a subaquatic daddy longlegs the size of a ball of yarn, elegantly arching its articulated limbs through the water in slow motion, coming to rest in your hand.

Now imagine not freaking out while it sits there. That's the kind of peaceful immersion Sea Trek produces. I began to understand how divers could spot "harmless" sharks 20 feet away and not thrash madly to the surface: When you're in the midst of all that bounty, you feel like just another denizen of the deep, but one big enough to be of no interest to feeding creatures. I was glad one of those massive groupers didn't happen by, though.

After the sea spider and the trumpetfish, holding a rust-colored sea urchin on my bare palm seemed natural, even when its hundreds of tiny fingers attached themselves to my skin. When the guide gently pulled it off, the creature yielded like reluctant Velcro.

We reacted pretty much the same way when it was time to return to our lives at the surface.

For a photo gallery with addition- al images of the Caribbean islands, go to www.washingtonpost.com/travel.

St. Thomas offers many ways to explore the sea, such as walking the underwater coral trail at Coral World Ocean Park. For a quiet moment, hit St. Thomas's Magens Bay beach in the morning. On the Atlantis submarine, guests can become submerged in St. Thomas.