In an effort to help relieve the region’s heavy airport congestion, the Bloomberg administration is quietly pushing forward a plan to put the city’s rivers to use as runways, a city official said yesterday.

As heightened security regulations at airports and overcrowded airspace add to flight delays and threaten to restrict the area’s economic growth, several options are on the table for easing the city’s air traffic congestion.

A spokesman said one option Mayor Bloomberg is throwing his support behind is a plan that could replace some airline shuttles that fly to and from East Coast cities with seaplanes — fixed-winged aircraft that take off and land on water — which could be used to pull traffic off overcrowded runways at the three regional airports that serve New York.

About 32% of arriving flights were delayed at La Guardia Airport in 2006, according to statistics from the federal Bureau of Transportation, as were 28% of arrivals at John F. Kennedy International Airport and 34% of arrivals at Newark Liberty International Airport.

Officials from the city’s Economic Development Corporation met in March with representatives from Tigerfish Aviation, an Australia-based seaplane manufacturer, to discuss how commercial seaplane service could work in the metropolitan region, and what kinds of planes could be employed.

The chief executive officer of Tigerfish, Saxon Ruddick, described the meeting as “very positive,” and the Economic Development Corporation is expected to reach out to other city agencies next month to discuss the feasibility of the plan, according to a city official.

The city is also considering a request from the New York City’s only seaport, the Skyport Marina at 23rd Street in the East River, to help it attract private investors and boost service. The charter planes that use the marina, whose flight routes are designated by the Federal Aviation Administration, are in highest demand during the summer months, when charter companies provide direct service to the Hamptons for about $350 one way, a marina spokesman said.

Commercial seaplanes have not been used in New York since the 1940s, when they would land at La Guardia Airport.

Moving air traffic to city rivers and harbors from airport runways is meeting with some skepticism at the Port Authority of New York and New Jersey, the agency that owns and operates the three major airports in the metropolitan region and earlier this year announced its plans to purchase a fourth hub, Stewart Airport in Newburgh, N.Y.

“The issue here is not so much easing congestion on runways — it’s an airspace issue,” a spokesman for the Port Authority, Pasquale DiFulco, said. Planes landing on water would still share airspace with planes landing on the ground, he said.

The three commercial airports in the region last year serviced 104 million passengers, and they are projected to service about 125 million passengers annually by 2025. The agency’s plan to meet this growth is not to throw more planes up into a crowded airspace, but to accommodate larger airplanes, Mr. DiFulco said. Meanwhile, even the largest seaplanes could carry only about 100 passengers a flight.

Harbor advocates, however, say the river is one of the city’s only underused transportation corridors. “There’s a lot of underutilized capacity on our waterways across the board,” the director of programming and operations of the Metropolitan Waterfront Alliance, Carter Craft, said. “But there’s a quality of life concern — seaplanes are certainly louder than boats.”

Seaplanes are not the city’s only venture into experimenting with regional transportation on its rivers. Two new ferry landings, at 90th Street and at 64th Street along Manhattan’s East Side, are being constructed by the city’s Economic Development Corporation, stimulating talk of a new high-speed ferry service to Atlantic City from Manhattan, according to Mr. Craft.

“I think it’s a good thing that the Mayor is thinking outside the box,” City Council Member John Liu, who heads the Council’s transportation committee, said in an interview yesterday. Mr. Liu noted, however, that if the plan went forward with planes that could land only on water, not also on land, it could backfire and create more crowding. “Strict seaplanes would be difficult because they’d have to have a destination airport where seaplanes could land, and that’s not really going to help us here,” Mr. Liu said.

While small aircraft were banned last winter from flying over the East River after a pitcher for the Yankees, Cory Lidle, crashed his airplane into an apartment building on the Upper East Side, seaplanes are still allowed to fly over the rivers.

The Federal Aviation Administration yesterday declined to comment on the city’s preliminary plans to boost seaplane transportation.

“There has been some discussion on the use of seaplanes for East Coast travel and potential service to JFK and other regional airports,” a spokesman for Mr. Bloomberg, John Gallagher, said, noting that the discussions were in preliminary stages.

The mayor first floated his seaplane strategy last June on his weekly radio show, during which he lauded seaplanes as an environmentally friendly mode of transportation. “You could land out away from everybody and then taxi in,” he said last summer, describing his vision for seaplane transportation in New York City.