The helicopter that plummeted into the Washington Channel early Friday plunged into the Main Street of a tightknit neighborhood of boat owners who live with the constant thunder of low-flying aircraft, the smell of engine fuel and the danger of disaster.

As investigators dismantled the remains of the Bell Jet Ranger, looking for clues to the crash that killed three passengers and injured the pilot, channel residents described the annoyances and risks that go along with life on the water.

"We're used to living with emergencies," said Richard Wellens, a marine engineer who has lived on the channel eight years. "There's always the danger of a fire, or the chance of an explosion or of sinking."

Living in bobbing yachts and barges strung alongside the restaurants of Water Street SW, channel residents said they are accustomed to the roar of flights in and out of nearby National Airport, the heavy rumble of military helicopters between the Pentagon and Bolling Air Force Base and the clatter of smaller choppers carrying photographers, dignitaries and sightseers over the city.

"That's the typical background noise of living here," said Richard Frey, who has lived in the channel seven years. "You can feel the vibrations in the water."

But the noise of a hovering helicopter Friday morning was also a typical sign of trouble. Usually the sound telegraphs the presence of a police helicopter over the scene of an accident, possibly a neighbor in need or a danger to other boats.

"You're always listening to noises and for changes in noises and smells . . . for smoke or fire or fuel, or for engine failure," Wellens said aboard his home, a 48-foot Chris-Craft cruiser. "When you're sitting on 300 gallons of fuel, you always have to be thinking of what's going on."

Friday morning, the noise was that of a chartered helicopter hovering 200 feet above the water to enable free-lance photographer William S. Weems to shoot pictures for a real estate brochure.

Witnesses said they heard the engine stop abruptly, and then heard the sound of a loud splash at 7:30 a.m. Weems, 44, of Northwest Washington, and the other two passengers, Robert Joy, 45, of Northwest Washington and Victoria Hinckley, 24, of Alexandria, were killed. The pilot, Jack C. Turley, 37, of Baltimore, was in serious but stable condition yesterday in the intensive care unit of George Washington University Medical Center, a spokeswoman said. Turley was suffering from cracked vertebrae and was being treated in the intensive care unit.

Turley was visited yesterday by Rus LeBlanc, the California yachtsman who rescued Turley by pulling him into his dinghy. LeBlanc, who spent several minutes at Turley's bedside, said the helicopter pilot "looked good" and thanked him for coming to his aid.

"He said he wished he could have met me under better circumstances," said LeBlanc, a real estate broker who is on a three-year cruise aboard his 38-foot sloop.

A team of three National Transportation Safety Board investigators studying the wreckage of the helicopter yesterday had not determined the cause of the accident, said Alfred Dickinson, the investigator in charge.

As mechanics worked to remove the C20 turbine engine, which suffered little damage in the crash, Dickinson said they planned to send it by truck to the Indianapolis plant of the manufacturer, Allison Gas Turbine, a subsidiary of General Motors Corp. There the engine would be tested as soon as possible under safety board supervision, he said. "We're in a rush on this one," because of the danger of salt water damage to the engine, Dickinson said.

Dickinson said mechanics had not determined whether the engine failed, but he added that "the preponderance of witnesses indicated that there were engine problems."

The crash caused little damage to the helicoptor's main and tail rotors, mast, transmission or engine. But the tan body lay crumpled and twisted on a barge on the Anacostia River, tied to the U.S. Army Corps of Engineers' wharf, near the Washington Navy Yard.

"Most of the damage, unfortunately, was to the cabin area where the people were," Dickinson said.

The number of deaths could have been much worse if the helicopter had been hovering less than 100 yards east, over the floating homes.

"We were . . . lucky it hit the water," Frey said, complaining that many helicopters needlessly fly over the boats. "But it's part of the responsibility you assume for living down here . . . . The law of probabilities leads you to expect that something is going to happen. You just hope it won't happen over your boat."

"We're sitting ducks," said Sue Longacre, who lives on a 48-foot Chris-Craft cruiser and said she has long worried about the helicopter traffic. "Talk about flammable. This is a wooden boat. I've often thought it was very unsafe."

Because of the risks and problems of living on the water, marina dwellers learn to depend on each other.

"There's a lot of camaraderie, but relationships are also based on the need to know each other, the need to get help from each other," Wellens said of his neighbors, a collection that includes eccentrics, swingers, yuppie mariners and a sprinkling of genuine old salts. Several boarded dinghies within seconds of the crash and were among the first rescuers on the scene.

Despite the noise and the danger of low-flying aircraft -- as well as the shipboard discomforts of cramped quarters, leaky cabins and faulty bilge pumps -- marina residents were unanimous yesterday in their enthusiasm for living on the water, where the freedom to travel can be enhanced by the comforts of luxurious cabins equipped with mobile telephones, microwave ovens and videocassette recorders.

"There are risks you take going up in a helicopter or living on a boat," Wellens said.