The one-fare plan fits with the liberal agenda of Mr. de Blasio, who has championed “transit equity” for all New Yorkers. To fulfill the mayor’s promise, the city will have to contribute a substantial operating subsidy, a commitment that several of his predecessors were unwilling to make.

Mr. de Blasio’s former rival for the mayor’s job, Christine C. Quinn, applauded his embrace of ferries as a form of mass transit. “There’s a little bit of a whimsical, historic notion of ferries; they seem to be a lot more fun than other modes of transportation,” said Ms. Quinn, the former City Council speaker. “You don’t want ferries to just be the fun, fancy transport of people with money.”

Of course, New York’s waters were once clogged with ferries. In the early 1900s, when there were few bridges and no car tunnels, as many as 147 boats carried people across the Hudson and East Rivers.

The only vestige of that era is the Staten Island Ferry, nine hulking boats that make regularly scheduled point-to-point crossings of New York Harbor. For routes from Brooklyn and Queens, city officials have largely relied on private companies operating their own ferries to deliver workers to Manhattan every weekday.

Image A design for a proposed Hornblower ferry. Credit... Incat Crowther

City officials have been leaning on Hornblower Cruises and Events, the San Francisco-based company they chose in March to operate the service, to order the boats it will need. Hornblower, which runs cruises to the Statue of Liberty, has settled on a design for 149-passenger boats and is negotiating with a few boatyards around the country to build 18 of them, at a cost of nearly $4 million each.