FALL IS BEDTIME for summer dreams. But sometimes on warm Indian summer nights they creep back to tease you like children who won't go to sleep: You should have moved out of this cramped downtown apartment. You should be floating free on the Potomac in a houseboat. You could still do it.

A few months ago, my summer dream was an obsession. It came to me full-blown one workaday evening when I saw a houseboat stealing out of town. The couple aboard had cast off their moorings like harness and were easing downstream on the tide, slipping into the biggest bathtub imaginable after a tough day at the office. I had a vision of a whole new way to live, and I was hooked.

The next day I started hanging around marinas. I became an expert at spotting boats people lived on, and sat watching them long after their weekend neighbors -- the My Ways, the For Plays and Vitamin Cs -- had struck their red-martini-glass ensigns and gone home.

I found houseboat lights glowing in the dark from the urban basins along the waterfront to secluded, suburban coves far down the Potomac. Hundreds of people were floating out there, and soon I began buttonholing them as they came through the marina gates. Then I was spending my weeknights on the dock -- one foot on the platform and one foot on someone's houseboat -- meeting folks like Bob Wilkins, a boyish 40-year-old who works at the Pentagon but by 5:30 is back at the Gangplank Marina behind Hogate's.

Every 20 minutes until 6:30 he has to dash out to a parking meter along Maine Avenue, pumping in quarters to hold the spot for his van, a beat-up 1960 Volkswagen that is both car and garage. Inside, he has stored whatever he can't fit into his boat.

For five years, Bob and his wife have lived behind Hogate's on a 47-foot Boatel. His houseboat, with that tell-tale barnacle color, is larger than many. Some of the neighbors' are so small that racks of clothes bulge out the back door.

His living room is also his kitchen, dining room and conning tower, with the steering wheel and throttle hidden by boxes. The "downstairs" has a bathroom plus a bedroom/basement.

That 220 square feet of total living space worried me, because my dream boat should be bigger. But it helped when he said, "Maybe I shouldn't be talking to you. I wouldn't want everyone to find out about this and move down."

On weekday evenings and Sunday mornings, the marinas belong to the live-aboards. Saturday evenings -- well, I stopped going down Saturday evenings. That was when I saw a guy, who if all went well would be me, tied to his slip six feet from a muscle boat that the weekend kids were pumping into a primal roar, a heaving, bucking red torpedo covered with searchlights and foghorns and 800 horsepower in back where a nam ought to be. I couldn't watch.

So I only went down on weekend evenings after the crowd at Gangplank had distilled to a village of a hundred people.

In the twilight, a magical thing happens. The boats solidify into a huge raft floating in the basin, a massive platform shifting as slowly as water in a scupper. All of midday Dupont Circle is aboard, condensed into just these townfolk loping along the boardwalks and asking about each other's houses: single men and women, couples and divorc,ees, children, gays, whites, blacks, civil servants, military . . . .

Over on Ida Ho is Representative Larry Craig of Idaho, in a houseboat the size of a Monte Carlo yacht but with the lines of a boxcar. On After Hours Two is Jack the Hook, living like a spider in a web. When I talked to him, Jack was sitting in his deep-sea charter berthed beneath Hogate's seawall and saying that if it weren't for all the strollers along the wall he would have given up marina life years ago.

One reason is the unpredictability of living on water. On a stormy night, you're sound asleep, bouncing in place, when suddenly your neighbor's house jumps up and mounts yours. Pots and pans are crashing in the kitchen, glasses are shattering, and outside the whole marina is screaming and trying to tie down the raging beast.

Maybe that was just one of those greenhorn stories for landlubbers like me about other landlubbers who went to sea and forgot to tighten their moorings before a storm; or substituted cheap automobile ignition parts for expensive marine ones, so that one night the parts corroded and the boat started by itself.

I hung in there -- until I started hearing about winter.

Because of winter, Bob Wilkins had been anxious, even in the close, spongy heat of the first evening I met him, to get back into the bowels of his boat. He was wedging fuzzy packets of insulation between the beams of his bathroom the way you might pad the attic, because last year he and his wife nearly froze to death. They had two electric and two kerosene heaters but they could only keep warm from the waist up. As fast as the warm air would rise, the cold Potomac wind blew in through his thin Mississippi houseboat sidings.

Insulating is a Sisyphean task, since houseboats were never intended for winter residences. The same is true for most of the live-aboard sailboats and power boats in the marina. Come November, when the Potomac turns icy and the north wind whips up the sleet and the rain, they're out of their element.

A really cold winter's day at the marina won't see anyone moving except Roger Rozelle, a Kodak salesman who inhabits a 34-foot catamaran. Below deck, he has less space than the world's smallest efficiency: two hollow pontoons and the bridge space between them. The kitchen and closet are in one pontoon; the bathroom and storage space are in the other. Between them, Roger entertains by roosting on his bed while his friends sit on pews beside him.

Roger says there's nothing like sitting outside on the fantail on a snowy night, watching the snowflakes streak across the lights of Washington while the gulls swoop and dive.

I tried to imagine it -- a thickly swaddled figure pretending I was far out to sea -- and failed.

There is an exception to all the freezing in the marina, and it gave me a last straw of hope. It is a floating chalet at the top of Roger's dock, right under the hulking Hogate's seawall -- a shimmering mountain-vacation cabin. On Christmastide nights, Roger watches its double-paned storm windows shining warm and yellow over the glazed river.

The chalet, which belongs to Bill Taylor, is built on a barge. Inside there is enough space for a permanent bedroom, a TV room, Taylor's piano, a full kitchen and the black-tie parties he loves to give.

But then the crushing news. There are only eight floating homes in the marina because most of the live-aboards resent them. They were introduced as an experiment by the marina management, which expectedhe year-round residents to flock to them.

Instead, the live-aboards sneered at them, saying they didn't want to see any more houses changing the neighborhood -- no matter that for many of the residents the cost would be about the same. In monthly mortgage and dock costs, a floating house is comparable to a two-bedroom condo on Capitol Hill -- or about what Rozelle spends on boat payments and docking fees to live in the sausage hollows of his catamaran.

I let my houseboat vision sink, but I still linger by its watery grave. My mind keeps going back to the marina, wondering what I missed, what holds the live-aboards in thrall.

I'm beginning to think I know the answer. My dream may have only been strong enough to last into Fall. But now and then the live-aboards' fantasy actually comes calling; a yacht sails up the Potomac from the Mediterranean or Australia. The sailors, a couple in their forties, are like gods. They tie up just outside the marina, they pay no marina fees. But the marina gives them a key to the gate, lets them use the showers and tie their dinghy to the dock. The live-aboards take them out to dinner and show them their boats until the gods weigh anchor and sail away again. Sometimes, the dream is as close as Tim Ehlen sitting on the roof of his houseboat at suburban Tantallon Yacht Club near Oxon Hill.

Six years ago, while writing for the Smithsonian, Ehlen sailed aboard a Spanish navy schooner that retraced Magellan's planned circumnavigation of the world. One day again, he wants to sail around the world, only this time not with 400 other men.

That is the dream of Bill Taylor, too, in the floating chalet. He is 45 and manages the government office of a national computer company, but he's getting a mechanic's license and a Cordon-Bleu culinary diploma so he can someday odd-job across the seas in his sailboat. Roger Rozelle on the catamaran is planning the same thing; so is Bob Wilkins on his houseboat.

"Down that stream," sys Ehlen, pointing to the mouth of Tantallon's cove, "is the Potomac, and down the Potomac is the Bay, and then the ocean. That's the fascination of the water. It's fluid. You can't drive out of the city without having to stop at stoplights. But on the water, you can just float away."

There's the answer. I had heard the same Siren, but only for a few moments one summer. He has a lifetime infection.

Of course, only a few of the live-aboards finally vanish down the Potomac. Most drop out of the life to marry or remarry or have children. Perhaps the end comes as it did for the computer professional from India, whose wife by an arranged marriage arrived on an airplane and ran him ashore that day. Or maybe it comes with renting an apartment for just a few months to get you through the winter.

Still, you can come and dream like I did. The boat will be there, bobbing in happy limbo.