I'M STILL NOT CERTAIN who planted a recent issue of Skin Diver in my office mailbox, but I strongly suspect a colleague who's deeply into the underwater scene. In any event, after a few glossy pages I was persuaded that getting to the bottom of things with mask, air tank and flippers might have some merit.

Here was a pastime with everything: tropical beaches, gorgeous underwater scenery, treasure dives to sunken wrecks, spearfishing, stunning women in gumdrop-pink Lycra dive suits. Yee-haw!

Two days later, I'm waddling around like the Stay Puft Marshmallow Man with a buoyancy-control device, a weight belt, compressed air tank and graphite fins. Fortunately it's early morning, so there's hardly anyone around at the Tysons Corner Holiday Inn to see this humiliating poolside performance -- except one beach-boy type ensconced in a chaise. His nose appears to be buried in a copy of GQ. Yet when I turn my back I do hear a muffled chuckle.

"Okay, you're doing fine," says diving instructor Mike Haberman. "Just ease off here and let your feet touch bottom."

Haberman helps adjust my mask, which covers eyes and nose, and explains how to breathe through a mouthpiece connected to the air tank on my back by a flexible hose and a pair of pressure regulators.

"Just breathe continually through your mouth -- don't hold your breath. Start equalizing the pressure in your ears by pinching your nose and blowing gently as soon as your head is under water.

"Keep your hands by your side and use your fins to swim with. We'll swim gradually to the eight-foot side of the pool. If you have any problems, give me a thumbs up signal and we'll surface."

Haberman instructs me to purge all the air from my buoyancy control device -- a pneumatic jacket which allows one to descend to the desired depth -- and gives me a thumbs down signal.

"Let's go," he says. "Follow me."

Haberman arches gracefully and submerges, his swim fins glistening only briefly at the surface.

I follow, a bit more clumsily, and in seconds I'm gliding across the pool bottom. I pinch my nose as instructed, breathing deliberately through my mouth. There's plenty of air, just like topside; the feeling is pleasant.

I sight Haberman about 10 feet ahead. "Okay?" he asks with a hand signal. I flash back an "Okay" and he motions for me to follow him.

Already, this is getting to be fun. I swim along a black stripe on the bottom to the deep edge of the pool, turning back to retrieve a lead weight that has fallen from my weight belt. The water is blue-green, clean and warm. For a moment, I fantasize a school of tropical fish.

After a few trips to the surface for further instruction, we log about a half hour underwater. I would like to stay a little longer, but Haberman suggests that it's time to pack it in.

"That's about enough for your first time out," he says. "I hope you enjoyed this as much as I did. It was easy, huh? A piece of cake!"

" "A lot easier than I thought it would be."

"Well, good. Let's get you into a class and we'll have you out in open water in no time." CARD-CARRYING

The scuba (originally an acronym for self-contained underwater breathing apparatus) industry is essentially self-regulated. Dive shops normally provide tank refills and equipment sales or rentals only to those with a certification card (or "C card") issued by one of the five major training institutes: the National Association of Underwater Instructors, the National Association of Scuba Diving Schools, the YMCA training program, Scuba Schools International or the Professional Association of Diving Instructors. This last is the largest; PADI offers a comprehensive training program, and a number of PADI courses have been approved for college credit by the American Council on Education.

"C" cards are issued, by PADI and others, for a number of skill levels, starting with "open water diver" and continuing to "advanced open water diver," "master scuba diver," "wreck diver," and so on.

"We begin lessons in a pool, like you did," says John Wall, owner of the Dive Shop in Falls Church, a PADI-affiliated store. "Then, gradually, we work up to training in open water diving. Most dive shops in the area use the Millbrook Quarry in Haymarket, Virginia, for this. It's only about 45 minutes from the Beltway."

It usually takes about four classroom sessions, three pool sessions and four open-water dives over a 3 1/2-week period to get an "open water diver" certification, the minimum training level you'll need to rent or buy equipment.

"You could travel down to the Caribbean, for example, and rent air tanks and weight belts from a local diving facility as an 'open water diver.' We're very strict about certification. If you can't produce a 'C' card, or if you demonstrate a real lack of knowledge, we won't rent or sell you anything. It's that simple," says Wall.

What keeps the shop owners honest?

"For one thing, the fact that your estate would probably sue us for a substantial amount if anything happened," Wall says soberly. "Diving instructors routinely carry a million dollars worth of liability insurance, by the way." BELOW THE SURFACE

As many women as men are recreational divers nowadays. If you are in generally good health and have no serious medical problems (especially chronic sinus, ear or lung conditions or metabolic disorders such as diabetes), you can consider the sport. Before undergoing dive training, you'll be asked to fill out a brief medical history. If you have any doubt about your ability to cope with scuba diving, it would be wise to check with your doctor first.

Scuba divers generally remain submerged for less than an hour at a time at depths of no more than about 60 feet, thus avoiding physiological problems associated with great depths (although dives to 100 feet and more are possible).

Diving is always done using the buddy system (at least two participants in the water); going it alone is considered hazardous and foolhardy. PAYING THE TAB

Dive industry brochures make the claim that scuba diving costs about as much as tennis, skiing or golf -- a fair assessment.

A check with area dive shops reveals that an introductory lesson can be had for around $85, certification to the "open water diver" level $215 to $250.

You'll require a basic outfit of fins, mask and snorkel, weight belt, boots and gloves ($150 and up) before taking a certification course. After certification, you'll need: a dive suit of either the wet or dry type (one lets some water in, the other doesn't) to keep you warm and protected from cuts and scrapes; an air tank and regulator with instrument panel and compass; and a buoyancy control device. These will cost at least an additional $500; the sky's the limit when buying custom or top-of-the-line equipment, as almost any serious sports fanatic will discover.

Normally, you'll rent tanks and weight belts at the site where you intend to dive. You wouldn't tote such heavy gear -- to Miami, say -- for a vacation diving jaunt. SPEAKING OF DIVING

Washington's near a number of choice dive sites, including Haymarket's Millbrook Quarry (see box). Also beckoning to scuba aficionados are the sunken hulks of two German U-boats off the Carolinas, as well as numerous shipwrecks near Ocean City, Maryland. Likewise popular with divers is the liberty ship John Morgan. It sank during World War II, spewing a cargo of Sherman tanks onto the seabed off Virginia Beach, where they remain.

"Wreck diving is really exciting," says enthusiast and instructor Al Yurek, "but it's generally a more advanced and deeper type of diving -- down to 100 feet or so.

"You can collect brass portholes and other artifacts, though -- and lobsters, too. The dive-ship captains know where there are wrecks with big lobsters crawling on them. I've gotten some that weighed five pounds, but there are bigger ones than that. It's like your own private refrigerator."UNDERWATER OUTTA TOWN

For those with clearer water in mind and a preference for reefs over wrecks, most dive shops offer year-round travel packages to exotic locales for less than you'd likely pay if you arranged the trip yourself.

"For as little as $375, you can spend the week with us in the Florida Keys," says Mike Haberman of The Dive Shop. "That includes food, lodging and the use of dive boats -- everything but transportation."

Sample prices for other shop-sponsored dive trips include $407 for four days and three nights in West Palm Beach; $879 for a week in Grand Cayman Islands, including airfare; or a 14-day trip to Truk and Palau in the Pacific for $3,284, airfare included. MEETING KINDRED SPIRITS

You won't have to travel far, though, to learn the basics of recreational diving; there are a number of local dive shops and clubs around.

Dive shops are the hub of scuba activities areawide, having taken on definite club-like attributes. In addition to selling or renting equipment, they organize get-togethers, training classes and trips, and many patrons are fiercely loyal to individual establishments.

"The good news," says one scuba veteran, "is that there are no bad dive shops in the Washington area." All are affiliated with one of the national certification groups and provide adequate training. The trick is to call around and find one that's convenient and offers a training schedule you can live with.

Clubs, while attracting fewer adherents than the shops, have their own appeal.

"You meet a more diversified group of people at the meetings," says Dale Fox, a dive shop employee and membership chairman of the Capital Divers Association. "They've been trained by different instructors, use various makes of equipment, and often have a perspective differing from divers trained at the same time and place."

CLUBS CAPITAL DIVERS ASSOCIATION -- In addition to members' training, the club sponsors trips to Florida, the Carribbean and other locales. Also featured is an active nautical-archeological division for those interested in exploring and preserving underwater sites of historical interest. Meets at the Naval Research Lab in Washington on the second and fourth Mondays of the month. Dues are $25 a year, with a $5 initiation fee. Mail inquiries to 10700 Marlborough Rd., Fairfax, VA 22032, or call Dale Fox at 591-8053. ATLANTIS RANGERS -- Founded in 1959, the Rangers offers specialty dive training and outings to wreck sites and other underwater East Coast points of interest. There are occasional jaunts farther afield, and underwater photography buffs are welcome. Meets on the second Friday of the month at the Greenbrier Apartments Recreation Building in Greenbelt. Dues are $20 a year, $7 initiation fee. Family memberships are $29 a year. Mail inquiries to PO Box 210, College Park, MD 20740 or call 703/578-3012. UNDERSEA ADVENTURE SEEKERS -- Dating back 29 years, this is the oldest dive club in the U.S. with a mostly black membership. Founder A. Jose Jones teaches marine biology at the University of the District of Columbia. This is a "serious" dive club; you must actually dive to remain a member. Dues are $60 a year, refunded in the form of various services. There's an annual trip to the Caribbean and journeys to Australia, Fiji and the Solomons Islands are planned this year. Meetings are rotated among members' homes and are held on the second and fourth Saturdays of the month. Mail inquiries to 1605 Crittenden St. NE, DC 20017 or call Dr. Jones at 526-3404. SCUBANAUTS say they're happiest when wreck diving, mostly in the waters off Delaware and Maryland, but with junkets to Virginia and the Carolinas. There are non-diving get-togethers, too, including a Christmas party President Mike Kedda describes as "fabulous." Kedda, an accomplished spearfisherman, says there's nothing quite like a meal of tautog or lobster freshly caught near a wreck site. Dues are $20 a year, with meetings on the second Tuesday of the month at the Diving Unlimited Shop in Gaithersburg. Write to PO Box 1801, Rockville, MD 20850, or call Mary Murphy at 933-6220.

DIVE SHOPS AMERICAN WATERSPORTS -- 6217 Livingston Rd., Oxon Hill. 567-2200. AMERICAN WATER SPORTS -- 6775 Wilson Blvd., Falls Church. 534-3636. CASSIDY'S GREAT ADVENTURES -- 7228 Nathan Ct., Manassas. 703/361-3483 (from D.C. area: 631-1191). THE DIVE SHOP -- 2841 Rogers Dr., Falls Church. 698-7220. DIVING UNLIMITED -- 15932-A Shady Grove Rd., Gaithersburg. 921-9777. DYNAMO DIVE SHOP -- 8906 Rhode Island Ave., College Park. 474-6380. NATIONAL DIVING CENTER -- 4932 Wisconsin Ave. NW. 363-6123. OLD INLET DIVE SHOP -- 2204 Highway 1, Dewey Beach. 302/227-0999. THE SCUBA SHOPPE -- 1053 E. Gude Dr., Rockville. 762-6200. SEA VENTURES INC. -- 9444 Main St., Fairfax. 425-7676. SKI & DIVE SHOP -- 1543 N. Quaker La., Alexandria. 998-6140. SUNSPORTS -- 18th Street, Ocean City. 301/289-3808. WET ONES DIVE SHOP -- 163 Glyndon St. SE, Vienna. 938-1111.