NYC Ferry launched its Astoria route Tuesday, punctuating a monthslong rollout of service around the city and highlighting the issues that lie ahead.

With four routes now in service, NYC Ferry has surpassed 1.5 million rides since its May 1 debut, Mayor Bill de Blasio announced Tuesday. There will be a break until the next expansion of two routes, the Lower East Side and Soundview, which are expected to launch next summer as City Hall grapples with a larger-than-expected demand.

“This is another great day for our city and it’s an especially great day for folks of Astoria, Long Island City and Roosevelt Island who are now going to have a lot easier time getting around,” said de Blasio, who greeted riders from the 6:30 a.m. inaugural ride as it arrived at the East 34th Street ferry landing.

The mayor boasted that the Astoria route would slash the commute to Manhattan in half for some residents and again pitched the service as an alternative to crowded subways and streets.

“Our waterways are nice and open,” he said. A one-way ferry trip is $2.75, though there are no free transfers to MTA subways or buses.

Ridership thus far has exceeded expectations — and has exacerbated issues for a service launched with an aggressive timeline.

To start, Hornblower, the operator of the ferry service, is still awaiting the building and delivery of six of the ferry network’s 20 boats. Without a full fleet, Hornblower has had to charter between two and four boats each day to meet ridership demands. Boats have been late and, especially on summer weekends, lines have been long for destinations like Rockaway Beach or Governors Island.

And since each boat has a 150-person capacity, packed vessels have left riders stranded at docks to wait for the next trip.

“Service has been good — except in the evening there’s a long wait. For the 5:15 p.m. or 6:15 p.m. boat, you have to get there a half-hour early to make sure you can get on,” said Suzy Karlin, a social worker from Long Beach who drives each day to Rockaway, where she takes a ferry to Wall Street/Pier 11. “There’s a frequency issue that needs to be worked out.”

Hornblower and the city’s Economic Development Corporation have adjusted their boat order to increase passenger capacity in three of the new vessels that will be delivered next year. Cameron Clark, Hornblower’s senior vice president for development of the ferry service, said the new boats will be able to carry more than 250 passengers.

With an anticipated 4.6 million trips annually, the city has budgeted a $6.60 subsidy for each passenger. The EDC doesn’t expect that subsidy to rise, even with the charters and boat order adjustments, according to spokesman Anthony Hogrebe.

Some transit experts are displeased with the attention and money being spent on a service that in one year will carry fewer passengers than the subway carries in one day, or what city buses could carry in two days.

“It’s always going to be marginal transit that serves people who live and work relatively close to the waterfront,” said Jon Orcutt, spokesman for TransitCenter. “But the bigger concern is that the mayor is so personally devoted to this whereas he has yet to show up to a Select Bus Service ribbon-cutting. He can directly influence the quality of the bus system,” which is losing ridership.

An overwhelming majority of NYC Ferry riders surveyed approve of their new commutes thus far, however. In an EDC survey asking riders to rate the service between 1 and 10, 93 percent gave a rating of 7 or higher, according to the EDC, which polled 1,300 riders.

“We’re going into communities that don’t have great access to mass transit,” Clark said. “That being said, we know we can continue to do better. So we’re going to work on all aspects of our operations between now and next spring. As we head into next spring, we’ll be ready to continue building on success with Lower East Side and Soundview.”