When the movie industry discovers us, I am lolling in shorts and sandals under a coconut palm on the Mexican island of Cozumel, nibbling a breakfast papaya.

With me is my friend Smith, who does something with computers back in Washington but who, after three days in Mexico, has forgotten why.

We have gotten as far as deciding against any major decisions during the day when up to the more or less curb of Cozumel's main street rolls a motoribike freighted with destiny. It is Rita Sheese, a SCUBA instructress and photographer of much charm last encountered two years ago leading divers into the deep.

"How'dja like to be in pictures?" she says, after many que pasos.

"We're on vacation," I say.

"No, really. I'm working with a movie company filming some underwater sequences next week and we need some divers to work as extras. Are you going to be around?"

Smith and I have been vaguely considering Belize or Honduras next week, but we're actually having difficulty thinking as far ahead as lunch.

"We're sort of going with the flow," I say. "Would the movie people furnish the tanks?"

"Oh, they'll furnish everything -- boat, tanks, everything you need. And they'll pay you."

"Who else is in the movie?"

"Farrah Fawcett-Majors."

I turn to Smith, who is carefully inserting his third cigarette of the still-young morning into his Smokenders filter.

"Well," he says, "if it will really help Rita out, I think we should definitely consider it."

The movie, it turns out, is something called "Sunburn," an $8-million whodunit spoof which opens with a speeding Cadillac plowing into a mariachi bar in Acapulco and exploding, showering tequila-soaked Mexicans, guitars, trumpets, furniture and chickens over the surrounding countryside.

The explosion forms part of an insurance swindle tackled by Fawcett-Majors and uart Carne0, who plays an elderly private detective in Acapulco.

At one point in the plot, in order to investigate a suspicious yacht, Fawcett-Majors goes SCUBA diving with a group of Acapulco tourists, which is where we come paddling into the picture.

"Sunburn" needs three minutes of watery footage in which Fawcett-Majors starts off underwater with the group, ventures off on her own under the yacht, gets attacked by one of the bad guys and is finally rescued. Keep Breathing

But in the world of movies, few things are what they seem.

Though the rest of the movie has been filmed in Acapulco, the underwater scenes will be shot off Cozumel where the water is clearer, and the reefs more spectacular than on Mexico's Pacific coast.

Fawcett-Majors, in a white wetsuit and white fins, has been filmed in Acapulco jumping into the water and getting out, but is presently in New York making Faberge commercials.

Her place underwater will be taken by a tawny Austrian double named Angelica Angely -- a veteran diver who once rode a killer whale while filming a documentary in the Sea of Cortes.

We extras, in turn, will be doubling for other extras already filmed in Acapulco suiting up for the dive.

"We're being called in as stunt men," says Smith, savoring the term. "Professional divers."

The explanation is not entirely accurate. We've been recruited -- like the other extras -- primarily for our proven ability to keep breathing and form part of rather ragtag bunch.

The others include:

Erlene Britt, a myopic Texan ("Cozumel is Houston's beach") diving for the first time without her prescription face mask. "I can't see a thing down there," she wails.

Gabriel Estrella, a diminutive Cozumel diving instructor once paralyzed with the bends after diving 275 feet down for black coral.

Candace (C.B.) Byers, a Georgetown member of the Washington contingent, who holds a masters degree in business and once was treasurer of a Vermont motorcycle gang.

"The way I look at it," says Smith, "C.B.'s overqualified for this."

We are supposed to look vaguely like our dry-land counterparts, Polaroid snapshots of whom Rita carries around on a clipboard. But with a mouthful of SCUBA regulator and a face full of mask, one actor looks pretty much like another.

Miguel Bursiaga, a curly-haired actor from Phoenix who rescues Fawcett-Majors from the underwater bad guy, for example, will also play -- with a mere change of trunks -- the underwater bad guy.

He will be doubling for the dryland bad buy -- a cowboy stunt man who falls out of airplanes and crashes cars for a living but apparently gets weak-kneed in the water.

Potential equipment discrepancies between the Acapulco and Cozumel segments, however, present a larger problem. I am issued black trunks flown in from Acapulco. Smith gets a wetsuit top but his own new two-tone fins are judged too flashy. Rita has 24 tanks repainted to match the aluminum blue of those already filmed in Acapulco. Our movie-issue face masks must be basic black. 2-2 Tango to Sea

Finally we put to sea on a 42-foot sport fishing boat named 2-2 Tango, which rumbles southward toward the reefs, scattering flying fish over a brilliant, sun-heated surface of cobalt blue.

"Tell me again," I say to Smith as we recline, tans deepening, on the Tango's flying bridge.

"They're paying us to do this," he says.

We go over the side first in Chankanab lagoon, where neon parrotfish and yellowtail school around us as Bursiaga leads us past Spanish cannons and an antique anchor rusting away on the ocean floor.

Divers in real life tend to scatter in pairs and peer into things, but we paddle past the camera in an overlapping 1-2-2-2-3 formation specially composed for visual effect.

The underwater action is directed and filmed by Ramon Bravo, a veteran underwater photographer known as Mexico's Jacques Cousteau.

Bravo crouches behind his suitcasesized camera as we swim past, his Jean Paul Belmondo features animated behind his mask. Periodically he bobs to the surface to give instructions. Rapture of the deep is not a problem: We're in maybe 12 feet of water.

After a couple of trial runs, I blow the first take by hitting an underwater light with a flipper. ("You kicked the sun," Bravo says.)

Other takes are confused by one of the other extras, a beach boy from the El Presidente Hotel, who keeps straying from his assigned position to get closer to the camera and, presumably, to stardom. He is eventually replaced.

Smith sucks dry one tank ("Love that air!") and gets another for retakes of his row of divers swimming away from Angelica. The rest of us blow bubbles on the sidelines.

Underwater it's a silent movie. Bravo directs us with gestures relayed through Bursiaga and through Rita, who carries stopwatches on a clipboard like some bubble-blowing key grip. Soon, we divers swim and circle in our formation naturally, much like the fish around us.

After lunch at the El Presidente we spend the afternoon jumping one by one off the hotel dock while Bravo films our splashy entry from underwater, and film-struck hotel guests watch in fascination from the thatchroofed bar nearby.

From the flying bridge of the 2-2 Tango, associate producer David Corda watches over the operation, his high forehead creased in an expression of perpetual concern.

Producers tend to be nervous people and Corda is no exception.

"I'm worried about that man," says Smith. "He's not savoring the moment." Beer on the Reef

"That's because he's paying for our moments," I say. Smith determines to subvert Corda's front-office attitude, however, and before the day is out has the producer diving with us on nearby Paradise Reef.

When the 2-2 Tango pulls out for the Palancar Reef next morning, investigation discloses an enormous ice chest stuffed full of Leon Negra beer Corda is one of us.

"A dry ship, I have decided, is not a happy ship," he says.

On the Palancar, we reassemble into our formation and wind among three-story-high coral towers as schools of pale blue fish ghost past.

Bravo films us from the jagged framework of a nearby cave peopled with multicolored sponges.

At one point, about 30 feet down, he has a silent, underwater argument with Angelica about the way to play the scene. There is much arm-waving and bubble-blowing. Finally, in obvious exasperation, he hands the camera to Rita and, with his eyes working expressively behind his mask, huddles behind a coral boulder and peeps out to show how her character hides from the group.

When the scene is complete we swim the tanks dry, chasing sting rays into a tiny underwater sandstorm while a 30-pound parrotfish lumbers along nearby.

Back on deck, as the boat heads back to shore, I find Smith, his cigarette filter cocked up like FDR's, musing in new directions. "The way I look at it," he says, "it seems a shame to quit this business just when we're getting the hang of it. Now Bravo says he's going over to Isla Mujeres to film some sleeping sharks and he'll need some help over there. Then in January or February there's this Charles Bronson movie where they'll need some divers...."