Life aboard a houseboat:

When you want a change of scenery, simply back out the boat, turn it around and dock the other end. From seascape to landscape, or vice versa, in minutes.

"It's a real sensuous way to live," purrs writer Tim Ehlen, 30, who has made Tantallon Yacht Club on Swan Creek, Prince George's County, his home for the past year.

A big minus (for some):

During the winter, "It was colder inside than out," says Dave Hall, 41, an executive recruiter who rents a houseboat at the Gangplank Marina, Water Street SW. "It was so cold the sponge froze to the kitchen counter top. I used three heat lamps, two kerosene heaters and an electric heater to get the temperature up to 60 degrees."

Many houseboats, says Gangplank dockmaster Don Reeves, are not made for this climate. "They're not insulated."

A constant worry:

"You're always scared your home is going to sink," confesses Cliff Ritt, 32, an insurance-company vice president, whose "houseboat" is, in fact, a converted lobster trawler that he's moving from Annapolis to Washington.

"One morning at 4 a.m., I thought I heard the boat gurgling. It wakes you up fast. Your first reaction is to jump overboard and swim for safety." (The noise was faulty plumbing.) "At 4 a.m., I was out on the dock with my hand wrenches."

Compensating joys:

"The other night," says Ritt, "we had dinner under the Bay Bridge at Annapolis. You've got the whole Chesapeake Bay and Potomac River as a back yard."

Perhaps as many as 300 to 400 people in the Washington area reside on a houseboat, sailboat or other floating home docked on the Potomac and its tributaries. About 150, including singles and many couples, call the Gangplank Marina home.

"Starting about three years ago, we had an influx of live-aboards," says Charles Mullaney of Anchor Away Houseboat Sales in Edgewater, Md. "Our sales have been good--the best in a number of years. We have people from every walk of life: singles, marrieds, doctors, lawyers, engineers. I know of a priest."

Most year-round boat dwellers are lured by the romance of the sea and a captivating life style. Another factor in the increasing popularity of boats: Shelter can be cheaper afloat than on land.

"I've had the idea for years," says Ehlen, a sailboat enthusiast from Detroit. Four years ago, on assignment for Smithsonian Magazine, he circumnavigated the globe on a Spanish navy four-masted training schooner, retracing Magellan's historic route of 1519-22.

For Ehlen, living on a houseboat means, "You don't share any walls with anybody. You feel less bound in--independent, freer. It's a very simple life. You get by with less. You can't own a lot of furniture and clothes. I like that."

He often anchors in a cove, returning to the marina at dawn in time for the commute to work.

"I went through suburban living," says Ritt, whose marriage broke up recently. "Two cars, a dog, commuting an hour and a half each way to New York for six years. I thought a boat would be quite an easy way to move into downtown Washington."

He paid $30,000 for his 12-year-old, 46-foot, steel-hulled vessel--plus $15,000 for mechanical repairs.

"I'm recently separated," says Hall. "I was faced with two choices. I could live in an apartment, which I haven't done for 10 years. Or I could buy a house, which I didn't want to do. So I found a boat I could rent." He pays about $325 a month, plus $45 a month for utilities, and enjoys the convenience of a downtown address.

A year and a half ago, Ehlen paid $17,500 for a 43-foot fiberglass boat built in 1974 by Nauta-line and abandoned by its owners at a Chesapeake marina. He spent $5,000 to repair the two gasoline-powered engines and to refurbish the cozy, six-room interior.

(A new single-engine, 42-foot fiberglass houseboat costs about $36,000, says Mullaney of Anchor Away. A second gasoline engine would add about $5,000. A 36-footer, thesmallest live-aboard he sells, goes for about $28,000.)

After an initial fee of $250, Ehlen pays $11 a month yacht-club dues, $86 a month slip fee, $30 a month for electricity and $10 a quarter for water. The boat contains its own sewage-disposal system. When not docked at a marina, a generator supplies electrical power, and the boat carries 100 gallons of fresh water.

With the mortgage, his total housing cost is about $425 a month, not including upkeep expenses or the cost of an occasional cruise on the Potomac.

A boating party up to the Kennedy Center (about 20 miles roundtrip) uses 20 to 25 gallons of fuel at about $1.25 a gallon currently. He tells his guests: "You bring the food; I'll bring the gas."

At the rear of Ehlen's boat is a guest bedroom (with a convertible couch) that doubles as a sunny, window-surrounded sitting room. Forward and two steps down is the small, but complete galley and a dining area that seats five. Adjacent is a tiny bathroom with shower. Two steps up again is the pilot room which Ehlen uses as a home office.

Under the forward deck, reached by another stairway, is the master bedroom, or "cuddy cabin." A king-size mattress almost fills the area, which is completely carpeted: ceiling, floor and walls. "It's like returning to the womb."

Closets are few, so Ehlen stores out-of-season clothes and other items in his Alexandria office. The washing is done at an onshore laundromat.

Much like any residence, a boat requires "a lot of maintaining," says Ehlen, who teaches an Open University course on buying and living aboard a houseboat. "I've learned plumbing, electricity, carpentry, hull work and fiberglass repair."

Ehlen was familiar with sails, "but powerboats were a whole different thing. It took me months to look at the engines." Once he did, he found working on them less intimidating than tuning up his car. "Bit by bit, I've conquered it."

Ehlen stresses to students the importance of regular tuneups that include checking the fuel lines. With a gasoline system, "The main danger is escaping fumes. There's an opportunity for a blow-up." Refueling calls for particular caution, a warning repeated by Coast Guard Chief Warrant Officer Roma Wade of the Recreational Boating Safety Information office.

Ehlen also has learned to switch off the master water source when he leaves for work. One day while he was away, a water pipe sprang a leak. By the time he returned, the bilge tanks had filled, and he stepped onto a carpet soggy from the overflow.

It's only a coincidence, he insists, that his girl friend also lives on a houseboat. Hers is moored in Annapolis, where she is a lobbyist. For most couples, Ehlen believes, a small houseboat is too cramped for a convivial relationship. "Many couples don't make a go of it on boats. People think it's romantic, but it requires a lot of work."

About 80 percent of the people who sign up for his class have never owned a boat. "I congratulate them for thinking of it. It shows a spark of imagination." A few are single females or young or retired couples. A large percentage are separated or divorced men.

"I think it's a dream for them, maybe."

Tim Ehlen's Open University class, "6 RMS RIV VU," is scheduled for Tuesday, May 11; Wednesday, May 19; and Monday, June 7, from 7:30 to 9:30 p.m. aboard his "Cruz Away" at Tantallon Yacht Club. Fee: $15. For more information: 966-9606.