Whoomp! Back into the drink again. This time it was a sudden gust of wind.

The time before, clumsy footwork had tripped me into an ungainly backflop.

And before that, headed straight at the dock, I jumped to escape a crash.

Learning to windsurf is a matter of doggedly hauling yourself out of the water and getting set up for the next plunge. There's always a next one.

As in any sport you're trying for the first time, you've got to have a sense of humor; but it's not hard to laugh when you see everyone else around you careening overboard. Anyway, on this hot September Sunday, a dunk -- even a dozen -- felt good.

I had joined a class of eight other novices -- "no sailing experience is required" -- who had paid $35 for the five-hour lesson taught by Robert Singer, a 27-year-old ex- accountant from Bethesda who one day filed away his ledgers to make windsurfing a career.

Back in 1977, after taking his CPA exam, Singer headed for Rehoboth Beach for a lazy summer. One day, he says, "I saw this guy sail up on this thing. 'Damn,' I told myself, 'this looks like a great thing to have.' "

Without even trying it out once, he bought a windsurfing board -- a "sailboard," it looks like a surf board with sails and costs about $930 for a standard racing model -- and spent the rest of the summer mastering it.

"It took me a good long time. There was no one around to teach me."

Two summers later he quit his comptroller's job with a shoe firm to go into business as Windsurfing Unlimited, selling sailboards in Washington and giving lessons here and in Rehoboth and Ocean City.

As the relatively new sport has gained popularity here, business has picked up. In the summer, five instructors assist him, and he estimates that a thousand people took classes this year. He helped form the 75-member Capital Windsurfing Club, which sails most weekends from spring through fall, and competes in an organized schedule of races that can lead to national and international championships.

With summer over, lessons now are offered (mostly through Open University) on Saturdays and Sundays through October. The site is a quiet, shallow cove on the Potomac just south of Woodrow Wilson Bridge, near Fort Foote in Prince George's County.

That's right -- the Potomac. If anything gave me doubts about showing up for class, it was the idea of gulping down a mouthful of its muddy flow.

So, the Friday before, I phoned Keith Brooks, an environmental analyst for the Interstate Commission on the Potomac River Basin, a watchdog agency that monitors pollutant levels.

"Is the river there unhealthy for swimming?"

"I would say no," Brooks reassured me. "The water quality should be quite good," especially, he pointed out, since rain had not fallen for several days. Runoff from a heavy downpour can increase the bacterial count.

Well, if he says so.

The morning session began promptly at 10. Dressed as instructed in swimming suits and tennis shoes (to protect your feet on the river's rocky bottom), we gathered around Singer for 45 minutes of dryland lessons. Class members, five men and four women, appeared to be in their 20s and 30s; at 44, I was probably the oldest.

"We're going to simulate on dry land what a windsurfer does on water."

In the parking lot, Singer set up a sawed- off sailboard that revolved about a foot from the ground. We took turns perched on this wobbly stand learning the basic techniques -- including where to put our feet and how to raise the sail without getting swept away.

"Windsurfing is about as hard as riding a bicycle," Singer coaxed, as nervous knees quivered even at this experiece, he reminded us, but "it helps," mostly becasue it gives you a feel for the sail in the wind. Still, the pair who claimed to be sailors, I saw, ended the morning as soaked as the rest of us.

Singer, slender and wiry with the deep tan from long days in the sun, had never sailed before his first summer of windsurfing. "I didn't understand the wind at all. And I have never surfed."

Now, with clear instructions to keep our backs to the wind -- "You can't sail directly into the wind" -- we trooped down to the dock for the real thing.

One by one we crawled hesitantly onto our boards, waiting for Singer to push us out into the river before we tried to stand up. He wanted us away from the dock, he explained, so we wouldn't hit it when we fell. And fall we did, like brightly colored tenpins, churning up the otherwise still surface of the cove.

The launching site seems a good spot for lessons, even if the murky water is uninviting. The cove is wide and lies outside the main river channel, so we didn't have to worry about smashing into power boats passing in the distance. The current, if there really was one here, was negligible. If we drifted, it was toward shore, not out to sea.

And, as we found out shortly, the cove is shallow. Instead of plunging over our heads, we usually ended up about waist deep, our feet well-braced in the muddy river bottom to make scrambling back aboard easier.

The first few minutes after our "fleet" -- that's what a cluster of sailboards is called -- raised sails was chaos. We plowed into one another, fell, got back aboard and rammed into someone else.

But eventually we began sorting ourselves out, taking wider sweeps out into the river, steadying our balance and practicing our turns. Move the mast a bit to the left, and the board heads up river.

Move it a bit back, and it turns downriver. Swing the sail across the board and step around the mast and you've changed directions by 180 degrees.

It seemed, actually, rather easy -- though we still flubbed up often enough to keep wet. The reason: The morning breeze had disappeared.

I had pictured myself clipping trimly through the whitecaps. Here I was drifting toward the muddy shoreline.

Don't worry, Singer advised. The wind will be back. Take the time to practice maneuvers. You'll need it when the breeze catches your sails.

He was right.

The wind began as ripples across the river. As it approached, our sails began flapping, and then they filled.

Whoosh! Mine was wrenched from my grasp as I tumbled after it.

Singer, suffering from a cold, yelled encouragement from the dock. An assistant scooted back and forth among us in a motor-powered liferaft barking out commands to shift sails to pick up speed or turn around.

Like a sheepdog, he kept the pack from scattering to Mount Vernon. Once he towed a board that had shot away from the group and was struggling to turn back.

Eventually, I began to get the feel of how to hold the sail and stay upright. As I took a quick glance around, I saw my classmates skittering across the river as delighted as I was. The breeze wasn't strong, but it was enough to give us the thrill that had lured us out that morning.

"This is could be fun," I thought, once I can cover 100 yards without slipping.

Five hours didn't make us skilled windsurfers, though it tended to rough up the knees a bit from all that climbing back aboard. But, I think, we got the basics on which to build.

If a day on the Potomac doesn't entice you (I think the turquoise waters of Bermuda, where I saw my first sailboard fleet, would be nice), Singer considers Rehoboth Bay at Dewey Beach the best spot for windsurfing in this area. The Capital Windsurfers often sail weekends at Sandy Point on the Chesapeake Bay.

Though the season for water sports is mostly over, the fall, he says, is an excellent time to windsurf. The water is still warm and the winds, usually, are better. "You're not into the sport if you miss the fall."

His season, however, may last into December with the help of wetsuits.

"Last December 7," he says, recalling an outing on the Potomac near National Airport, "was a fantastic day." If you're interested, you could probably arrange a class. GETTING ABOARD

For more information: Windsurfing Unlimited, 951-0705; Open University, 966-9606.