Before New York City Mayor Bill de Blasio delivered his second State of the City address, it was widely expected that he would focus the address almost entirely on housing policy. He did speak at length about his ambitious plan to build or preserve 200,000 units of affordable housing over the next decade. But it was a major transportation policy unveiled near the end of the address that surprised onlookers and made headlines.

“Transportation is central to the mission of providing affordable housing and services—connecting neighborhoods in the five boroughs to New York’s largest job centers,” said the mayor. Building these connections, he continued, could be achieved by taking advantage of the water with a five-borough ferry system.

This system would launch in 2017 with routes that connect Manhattan to Queens, South Brooklyn, and the Rockaways. The following year, ferries would run along Manhattan’s Lower East Side and between Manhattan and Soundview in the Bronx. Another route connecting Coney Island, Staten Island, and the Financial District is still in the planning stages. The administration has said that work is slated to begin this year on the $55 million process of designing and building the system’s docks; the city will also select private ferry operators to run the service. When completed, the ferries will accommodate 4.6 million trips a year, according to the mayor’s office.

Like many of de Blasio’s urbanism proposals, this one was born under his predecessor. In 2008, Michael Bloomberg worked with City Council Speaker Christine Quinn and the New York City Economic Development Corporation to create a framework for a citywide ferry network that includes many of the sites seen in de Blasio’s plan. The Bloomberg administration ultimately only moved forward with the East River Ferry. That service launched in 2011 as a pilot program and has been providing service between Manhattan, Brooklyn, and Long Island City for $4 a ride ever since. The ferry has been hugely popular, but still requires a significant subsidy—$2.22 per trip, according to a 2013 study commissioned by the EDC. (For comparison, there is a $0.62 subsidy for each subway ride.)

The de Blasio administration has said the new system would require between $10 million and $20 million in annual subsidies and that a ferry ride would cost as much as taking the subway or bus. Critics of the mayor’s plan say that the city’s money would be better spent on other transit programs like bus rapid transit that could reach lower-income New Yorkers who do not live near the water. (In his State of the City address, Mayor de Blasio also pledged to complete an additional 13 BRT lines.)

Creating and sustaining a viable city-wide ferry system—even with considerable subsidies baked in—will not be easy to pull off, said Jeff Zupan, a senior fellow at the Regional Planning Association who has been studying New York City ferries for decades. In that time, he has seen plenty of ferry attempts fail. Just last fall, the de Blasio administration discontinued ferry service to the Rockaways that was set up after Superstorm Sandy because it was costing the city about $30 a passenger.

Zupan is skeptical that the new Rockaway iteration—or any of de Blasio’s planned routes for that matter—will fare much better. To be successful, he explained, ferries must provide a quick and efficient ride between people’s home and office. This is most feasible when ferries run between densely populated areas (think Hoboken to Lower Manhattan) where it is easy to get to and from a dock and then onto a final destination. Short distances also make matters easier because riders are enticed with a faster trip and ferry operators can run fewer boats while still maintaining frequent and reliable service. Many of de Blasio’s proposed routes do not have this built-in advantage.

“They are not all going to be dogs,” said Zupan referring to de Blaiso’s planned routes, “but they do not have all the features you want to look for. If they had all the features, these would have been done long ago because these ideas have been around for a long time.”

But Zupan noted that the resurgent waterfront, with apartment towers sprouting up one after the other, has buoyed the mayor’s plan. The glossy buildings may offer great views, but are typically a hike from transit options. Citywide ferries could be a major boon to developers already eager to build near the water. The mayor’s office did not respond to AN’s question about whether it would ask developers to contribute funds for the ferry system.

Ultimately, the mayor’s five-borough plan is a kit of parts with only some routes seeming positioned to succeed. But what will happen to some, or all, of the ferries cannot be known until the boats hit the water. “You can never really know until you try it,” said Zupan.