Four years ago, when the youngest of their three children was approaching college age, David and Jennifer DeLancey began thinking about a home on the water.

They had both retired young and in the early 1990s built a 6,000-square-foot house outside Annapolis "to raise the kids in." But they couldn't see the Severn River or the Chesapeake from it, and now, facing the proverbial empty nest, they wanted to downshift to something smaller where they could sniff salt air.

But they found waterfront property obscenely expensive, and the shoreside condos they looked at seemed as if they were made of cardboard.

"I told David if he could give me three bedrooms and a full-size kitchen and bath I'd gladly move onto a boat," Jennifer says. So David, 51, did just that. And then some.

Today the DeLanceys live afloat on the Jennifer Ann. You could call it a houseboat, but that would be like calling Air Force One a plane. Within the sunny, air-conditioned confines of the custom-built, 65-by-20-foot, three-story steel hull, David has packaged more space and amenities than most apartments and condominums, and many houses. The 1,500 square feet of interior living space embrace 3 1/2 bedrooms (including a 14-by-20-foot master suite), 3 1/2 bathrooms, and a bright and open living/dining area (complete with a granite dining table) where the DeLanceys have comfortably entertained as many as 50 people to watch a Blue Angels air show or the colorful start of major yacht races on the bay.

Jennifer's designer kitchen boasts a full-size electric stove, granite counters, two dishwashers, a trash compactor and Sub-Zero refrigerator plus "as much drawer and cabinet storage as I had in my house outside Annapolis."

All this breezy panache is contained within a vessel the DeLanceys acknowledge is engineered more for utility than exterior elegance. The Jennifer Ann bears a closer resemblance to a downsized Cape May Ferry than to anything ever owned by Aristotle Onassis. That's all right with the DeLanceys. "We'd rather people shrug when we come into the marina, then come aboard and say, 'Oh, wow!' " says Jennifer.

Like many boaters, the DeLanceys can awaken in a secluded cove to the soft beat of heron wings, or sip cocktails amid sunwashed marshes while tossing peanuts to mendicant ducks. But unlike many powerboaters, who drive from one set of marina hook-ups to another, the DeLanceys and their vessel are engineered for a major getaway. With two 250-gallon holding tanks, 1,400 gallons of freshwater tankage and a fuel range of 1,200 to 1,400 miles, the Jennifer Ann can easily go three weeks without touching shore.

"It's not that we're antisocial," says Jennifer. "I always say there are no strangers when you're cruising, just friends you haven't met." Guests and children come and go from the Jennifer Ann as it pokes among the leafy green anchorages of coastal rivers and inlets that the non-boating public rarely sees. Jennifer keeps a photo logbook documenting where they've been and people they've met. But because they'd rather anchor away from dockside clamor, the Jennifer Ann can go days without starting the engine. A bank of eight marine batteries avoids the noise and necessity of running the generator daily to keep the refrigerator and other systems going. This, to David, spells serenity.

"We like to hear the marsh sounds," he says. "I spent a lot of time and money on insulation to make this boat as quiet as possible, even when we're underway."

The DeLanceys have traded their car and the traffic jams of Annapolis for a motorized dinghy with which they fetch guests and groceries and ferry their bicycles to the pine-shaded bike trails of sleepy waterway villages and island parks. They trade novels at marina book swaps, catch up on correspondence in their breezy main salon via cell phones and wireless Internet service, and have generally needed few adjustments in moving onto the water.

"I was really ready to live smaller," says Jennifer. "When you look after 3 1/2 acres and 6,000 square feet, you become a slave to your house." But as a trim and energetic former gym teacher of 54, she set one condition for becoming a liveaboard. "I told David he had to get me off the boat every day to get an hour's exercise. And he has."

The Jennifer Ann is driven from a top-deck wheelhouse. Two 320-horsepower Caterpillar diesel engines give the vessel a cruising speed of 8 1/2 knots. The two skeg-protected, three-blade props are mounted nine feet apart to maximize maneuverability. With the help of a bow thruster -- a transverse-mounted bow propeller that helps kick the bow to port or starboard independent of the rear engines -- the two DeLanceys can dock their three-story vessel by themselves even in a 20-knot wind.

"If it's blowing more than 20, we just anchor and wait for the wind to die," David says.

The couple say the secret of their contentment with the Jennifer Ann is that, unlike boats whose living conditions must conform to a pre-designed hull shape, theirs was built "from the inside out." David even shaped the main living area around a favorite semicircular couch. But there were some basic parameters.

"We wanted the outside length to be 65 feet, because beyond that you get into another whole universe of Coast Guard safety regulations, environmental systems and so on," he says. The 20-foot width was chosen to fit the largest marina slips generally available, and because, just as plywood comes in 4-by-8-foot sheets, steel comes in 10-by-40-foot sheets. A 20-foot width minimized cutting and wastage. They wanted a maximum height of 18 feet above water to permit clearance of low bridges.

David, who sold a steel-fabricating business outside Erie, Pa., before moving to Annapolis, says his familiarity with metalwork helped guide the design. He says the biggest challenge was distributing the vessel's weight evenly enough to permit it to operate in only five feet of water.

"We knew from our sailing that five feet lets you into a lot of places we wanted to go in the Chesapeake, plus it largely governs access to the Great Bahama Bank in the Caribbean," he says.

The two doodled with the design through most of 1999, then let the contract for the hull that December. The keel was laid in May 2000 in a shipyard in Oriental, N.C., and a year later the two drove what was essentially a bare but motorized hull north to Mears Marina on Annapolis's Back Creek. There, David wired, plumbed and otherwise engineered the interior spaces for another year while Jennifer decorated. In all, they estimate the Jennifer Ann cost them $900,000 plus their time and labor. It would cost at least $1.5 million to duplicate, they say.

They had agreed early on that they wanted more house than boat.

"This is an extremely safe and seaworthy vessel," David says. "It's got watertight compartments and all the safety features, but we're not about to take it around the world. We've designed it for mostly inland cruising, not because the hull couldn't take conditions offshore, but because it wouldn't be comfortable riding her out there. She has a flat bottom and no stabilizers, so she would ride a bit rough in extreme conditions."

They have already operated the boat handily in 30 knots of wind and four- to six-foot seas in the Chesapeake, thanks to their spare living and special locks on cabinets and doors.

As veteran sailors they knew the shelf-borne bric-a-brac and random counter objects of the average house become lethal missiles in a wave-tossed boat, but doing without them wasn't much of a sacrifice. They had owned and sailed a 51-foot Beneteau sloop from 1987 to 1995 and were conditioned to keeping things shipshape. Besides, Jennifer says, "I don't like a lot of stuff."

The payoff has been the freedom their new home provides. So far they have used the Jennifer Ann to meander down the East Coast on the dreamy Intracoastal Waterway, which they call "the best-kept secret in America." They love the coastal marshes, the quaint little towns in the Georgia and Carolina Low Country, and they say the elevated view from the Jennifer Ann's upper deck provides an expansive perspective denied to the average boater. In January, while the rest of us were chipping ice from our gutters, the DeLanceys and their nautical house were sunbathing in Key West. They spent much of February and March anchored among the coral islets of Islamorada in the Florida Keys.

But their vessel is not a traveling cure-all, they say. They still expect to park it from time to time and charter sailboats, fly to Europe or otherwise broaden their horizons.

"I tell people we finally have our home on the water," Jennifer says. "But now, if we ever don't like the neighbors, we can just cast off the lines and we're gone."

Style section writer Ken Ringle is master and commander of the schooner Whisper, which he sails on Chesapeake Bay.