City officials, who had argued against the need for bigger boats, relented and revised their order to include boats that can hold 350 passengers. Three of those larger boats are scheduled to arrive from a shipyard in Louisiana over the summer and three more are expected next year.

In the meantime, Hornblower, the company that operates the ferries, will charter as many as eight boats that can carry up to 500 passengers each, said James Patchett, chief executive of the city’s Economic Development Corporation. Mr. Patchett said the chartered vessels would allow the city to narrow the gap between boats to 20 to 25 minutes on all four of its routes, down from 25 to 60 minutes last summer.

But Mr. Patchett warned that he could not promise that demand would never again exceed the capacity of the ferries. “On a really crazy-beautiful day in the summer, when it seems like everyone in the city wants to go to the beach at the exact same time, there are still going to be lines,” he said.

The service is scheduled to add two routes this summer, one starting in the Soundview section of the Bronx and the other on the Lower East Side. After those routes are established, city officials will decide where the ferries might go next, Mr. de Blasio said. “There’s a lot of places that would like ferry service,” he said.

Mr. de Blasio disputed the idea that the investment in the ferry service would be better spent improving the subways and buses, which serve far more riders and some of the city’s lowest-income neighborhoods. He reiterated his view that the best solution for the subways would be a steady source of increased funding, such as an additional tax on the city’s wealthiest residents.

“The answer is a long-term funding source that would allow us to fix the whole God-forsaken thing,” Mr. de Blasio said, adding that he believed a different balance of power in the state Legislature this fall could provide sufficient support for a so-called millionaire’s tax.

Standing on the dock in the sunshine, Mr. de Blasio refused to let the steady tide of questions from skeptical reporters erode his enthusiasm for the ferry service he deemed a resounding success.

“We’ve got to do more than one thing to create a 21st-century city with multiple mass transit options for people,” the mayor said.