John J. McNally said he was sitting in traffic -- as usual -- one morning on his way to work in Crystal City when he "saw this big river out there and no one on it, and thought, 'This is crazy.' "

To McNally, a Fairfax County resident and transportation consultant, the Potomac River shimmered that day like an enormous empty highway on which commuters could glide to work by ferry boat.

Since then, McNally and his partner, Alan G. Gray, have persuaded several local officials that it's worth their time and even money to study the idea of getting cars off their increasingly congested roads by offering ferry service to Washington from Prince George's and Prince William counties.

"Transportation planners solve problems with bridges, roads and tunnels. They've never looked at the river as an alternative," said Gray, president of a government consulting firm in Alexandria and a resident of Fairfax. "They are now."

The commuter ferry idea, dubbed "Metromarine" by the two entrepreneurs, has generated both interest and skepticism among local transportation planners.

Supporters see the possibility of removing hundreds of cars off the traffic-choked north-south arteries on both sides of the river, while allowing commuters to exchange the headaches of driving for the pleasures of reading, eating and socializing on their way to work in the morning. And they ask, if ferries work in such cities as New York, Boston, San Francisco and Seattle, then why not in Washington?

Skeptics want to see more information about the costs of the project and the potential for obtaining financing, liability insurance and passengers.

"The notion of using the water because of its relative lack of congestion is appealing," said Stephen T. Roberts, director of the commuter rail project of the Northern Virginia Transportation Commission. "However, beyond that, nothing I've heard makes any sense."

"We are in favor of the concept," said Prince George's County transit administrator Dee Allison. "The Indian Head highway corridor is very congested, with traffic backups for miles and miles up to the Beltway to get to D.C. and Virginia . . . . As the traffic gets worse, we are looking for alternative ways to get to work."

District officials contacted had not studied the ferry proposal, but expressed interest. "It's something we ought to look at," said Madelyn Andrews, spokeswoman for the D.C. deputy mayor for economic development. "It doesn't sound farfetched."

The Metropolitan Washington Council of Governments plans to spend $20,000 to study the ridership potential for a ferry service. The study will try to estimate how many people would ride a ferry to work, depending on the travel time, the cost and the location of departure and destination points, said Ronald F. Kirby, COG's director of transportation planning.

The COG study, while not an endorsement of the commuter ferry proposal, is "a sign that it's certainly worth looking into," Kirby said.

The Potomac Rappahannock Transportation Commission, the transportation planning agency for Prince William and Stafford counties, Manassas and Manassas Park, intends to use the COG ridership findings to conduct a feasibility study, examining issues such as costs and possible sites for ferry terminals, said Joy Schaad, acting director of the commission and Prince William County's chief of transportation planning.

"We're kind of reserving judgment until we see the results of the feasibility study," Schaad said. "But if you think it's a totally stupid idea, you don't even put money into a feasibility study."

McNally and Gray say the idea can work as a "private-public partnership," a phrase that has become a transportation buzzword in an era of tighter government budgets. It means the costs would be split between business and government.

Under the proposal, their company, Metromarine Transport, would handle the waterborne operation while local governments would pay for the land-based facilities.

The company would buy at least two boats and operate a service ferrying hundreds of passengers each morning and evening between Washington and points as far as Woodbridge on the Virginia side and Marshall Hall on the Maryland side. A one-way trip from Woodbridge to the District would take about 50 minutes; a round-trip ticket would cost $8 or $9, according to preliminary estimates.

During the non-rush hours, the boats could take tourists to Mount Vernon or shuttle travelers from National Airport to the District's Maine Avenue waterfront, Georgetown or the planned PortAmerica development in Prince George's County, according to McNally and Gray.

The routes have not yet been determined, but other possible sites for ferry terminals include Alexandria and the Pentagon on the Virginia side, Fort Washington in Maryland, and the future Navy Yard Metro station in the District, the promoters said.

They said the venture could operate profitably, with no need for additional public subsidy, if local governments paid for construction and maintenance of the ferry terminals, which would include piers, parking lots, waiting lounges and possibly bus service to the docks. They have not estimated the terminal costs.

McNally and Gray estimate the company's start-up costs at $1.5 million to $3.5 million for each boat, $2 million a year for labor, fuel, maintenance and other operating costs, and up to $50,000 a year for insurance.

The routes and type of boat will depend on the results of the ridership study, they said. "You don't want a boat if you can't fill it up," McNally said, estimating that the vessels would have to operate about 70 percent full to make money.

They have begun looking at boats capable of carrying 130 to 250 passengers, traveling at 30 to 40 knots and able to navigate through shallow water and under bridges. They have ruled out hovercraft as too noisy and too expensive to maintain and operate.

Metromarine plans to raise the initial financing from private investors, "just like any start-up company," said McNally, a former Navy pilot.

The company could begin service within a year after reaching an agreement with local governments and raising the start-up money, he said, acknowledging that it is difficult to estimate how long the local governments might take to study the issue and decide on financing the terminals.

Zeanious Newcomb, a Stafford County resident and member of the Potomac Rappahannock Transportation Commission, has talked with transportation officials in other cities with ferries and said, "It seems to be working well under the appropriate circumstances."

One concern, he said, is finding a way to keep the boats in business during the day. "Tourism is good in the summer, but in wintertime it's a different proposition," Newcomb said.

Plans for PortAmerica already include facilities for a water taxi to carry travelers between the development and the airport, said developer James T. Lewis. "It's a very important part of the project."

"It might be quicker, especially at rush hour, than driving across the Woodrow Wilson Bridge and through Alexandria," Lewis said. "It's curious that the river isn't used more for transportation."

Many local officials said they have to look at all possibilities for getting cars off the north-south highways.

"Congestion and commuting are getting to be such a problem, and all the studies show it getting so much worse, that we need to pursue every opportunity to meet those unmet travel needs," said Schaad of Prince William. "We're looking at everything at this point."