One morning two months ago, Carol Collins awoke in her boat in the Gangplank Marina, looked out the porthole and had an alarming vision: She saw a house.

Not a houseboat, you understand, but an actual house, complete with front door, cedar siding, powder room and optional trash masher, floating less than 50 feet away among the Bayliners, Chris-Crafts and sloops of Washington's Southwest waterfront.

"It was big as a barn and blocked my view of the water," she said. "I couldn't believe it."

For Collins, a free-lance writer and editor who fled the psychic confines of apartment living three years ago for the sun-soaked Margaritaville of the Washington Channel, the house bore all the ominous portent of a creature from an alien world.

"It looked," she said, "like something from the suburbs."

Today the floating house and its siblings (there are now five) have drifted into the eye of a ministorm, touching such weighty questions as the future of Washington's waterfront and the economic and cultural conflicts of its businesses and residents.

Defenders and critics alike see in their arrival the prospect of a semi-Sausalito on the newly clean Potomac--a waterborne subdivision of balconied split-levels where urban planners had envisioned motor yachts and masts.

"This is a marina," said John Dunning, a retired association executive who lives on the 36-foot sloop Western Star. "It's not a damn housing development."

"There's room for everybody on this waterfront," says Pat Byrne, partner in the Gangplank corporations, which sell the houses and run the marina. "Nobody should try to force a life style on anybody else."

In one sense the floating debate itself signals victory of a sort for everyone. Two decades ago the Potomac flowed foul with sewage, its only houseboats nailed together from plywood by a few Buzzard Point diehards in search of cheap living.

So barren and hopeless appeared the Washington Channel waterfront that at first no one bid for rights to build the marina sketched by planners of the new Southwest. As late as the mid-1970s, construction of the cross-channel tunnel for Metro's Yellow Line left the waterfront torn and choked with debris.

Byrne, then an accountant in the floating Gangplank restaurant, remembers it was 1977 before the first of the 370 piers and boat slips that had been proposed a decade earlier could be installed.

Today, amid the national waterfront development boom symbolized by Baltimore's Inner Harbor, the Potomac is suddenly prime real estate. Windsurfers and bass fishermen test their skills on waters long barely tolerable to carp. Just downstream, developers tout the multimillion-dollar Smoot Bay project designed to turn a long-forgotten Prince George's County cove into a major urban showpiece.

To the live-aboards at Gangplank Marina, such economic and cultural currents echo like the jetwash of outbound planes.

Dropouts from commuting and cul de sacs, most turned to life on the river, they say, for qualities they had lost or never had: openness, freedom and the illusion, at least, that one can always cast off the dock lines and sail away. The floating houses, they say, embody everything they left behind.

"If I wanted to look at those things," says Karen Caldwell, a two-year live-aboard at Gangplank, "I would go back to Largo."

"One of the things that strikes you when you live down here is your involvement with the physical world," says Collins, previously a nonboating Manhattanite.

"In a house or apartment you're insulated from the weather, for example, but here it really matters. You can feel the wind on the water. You wake up to the sun or rain. If a storm comes you have to tighten your dock lines. . . . It becomes a very personal thing."

The boaters' common vulnerability breeds a sense of community largely unknown in city living, she said. "We look after each other's boats," she says. "If there's a thunderstorm, I'll come home from work and find my deck chairs folded and put away where the wind won't get them."

What keeps more people from the good life on the river, boaters say, are two things: "Pullman living" in the cramped space of a 40-foot hull, and the winters, which in the uninsulated metal and fiber glass of summer-built boats can give a penguin frostbite.

The floating houses, which range in price from $50,000 to $75,000, are designed to overcome that. Built in Mays Landing, N.J., and shipped in by truck, they come complete with foam insulation, wall-to-wall carpeting and tile baths and dishwashers. Both one- and two-bedroom models have roomy closets and one has a sunken living room that, critics say, could acquire new meaning in the first big storm.

"The damn things aren't boats, they're houses," says Dunning, a graying veteran sailor. "The people who buy them won't realize how vulnerable they are. They won't take care of them, and they won't take care of us."

Pat Byrne disagrees. The floating houses, she says, are technically licensed vessels, no less mobile than many others at the Gangplank, some of which actually have sprouted bushes on their unused motor housings.

"We try to maintain a mix of sailboats, powerboats and houseboats here and that mix will continue," she says. Present plans call for locating no more than 10 or 12 of the homes at the marina, she added, and those will be located along the sea wall or possibly on a single dock "so as to cause as little disruption as possible.

"I can't believe that having these occupy 3 percent of the slips is going to change the character of the marina."

Byrne, however, acknowledges that the houses have moved the marina into uncharted waters. Others agree. Apartment and condominium residents elsewhere in Southwest have discussed what the advent of the floating houses might mean, and one of them, Dorothy Henry of Harbour Square, has asked the city, in effect, to say when a boat becomes a house.

In a June 17 letter of protest over what she called the "unsightly" additions to the waterfront, she asked the District Corporation Counsel's Office "whether 1) it is legal to put nonnavigable homes in the Washington Channel; 2) if so, what is the real estate tax liability of the owner, and 3) what action can be taken to have the existing structures removed to prevent further defilement" of the Potomac shore.

Henry said Friday she is still waiting for an answer. Even a negative response, however, need not crimp sales. Pete Bastien, one of the brokers at Gangplank Boat Sales, which sells the houses, says their real market may be at Smoot Bay, where more than 1,000 boat slips are planned in an area already essentially suburban.

Meanwhile, those who live in the houses tread softly around the sensibilities of their boat-owning neighbors, who have nicknamed the controversial structures "BDUs--big, dumb and ugly."

Robert Cirace, a baldish bachelor who owns his own television production company, rents one of the houses from Byrne and her partners for $1,000 a month and proudly shows visitors such glories as its loft bedroom skylights and rooftop deck.

"I get a few remarks about it, and I'm a little paranoid," he says. "I keep expecting to come home and find my dock lines cut or my phone disconnected. But most people are nice to me, no matter how they feel about the house. They just ask where the bow is. And when I'll take it out for a cruise."