As the city prepares to mark the fifth anniversary of Superstorm Sandy, a group of environmentalists say the city remains vulnerable to storms, rising sea levels and other natural disasters.

Large sea walls and sand dunes have been touted as accomplishments by the city in its resiliency efforts, but the coastlines are still vulnerable to devastation, according to Bill Golden, president of the National Institute for Coastal Harbor Infrastructure, a nonprofit working to storm-proof coastlines.

However, Golden said one proposal — a storm surge barrier — has not been getting enough backing. He and fellow activists took to the sea Tuesday to push for the high-tech structures.

He teamed with the Port Authority, MTA, Manhattan Borough President Gale Brewer and other environmentalists for a boat tour to look at coastal locations that took the biggest hits during the Oct. 29, 2012, storm.

“The impact of Sandy on the metropolitan area was so complex that the only way to understand it ... is to have a tapestry of experiences and visuals,” he said.

The resiliency proposal, which has been floated for years, would create a system of storm surge barriers at four spots: the Outer Harbor Gateway, between Sandy Hook, New Jersey, and Rockaway; the East River between Queens and the Bronx; East Rockaway, and the Jones Inlet.

Concrete structures would be installed at the locations and placed below sea level. Each structure includes a hinged steel gate that can rise to 92 feet from the water to create a barrier during high tide and surges.

“The gates are open 99.9 percent of the time,” said Malcolm Bowman, the Stony Brook University oceanography professor who founded the New York and New Jersey Storm Surge working group. “We keep back those storm surges associated with hurricanes and nor’easters.”

Like many of the other post-Sandy projects, the surge barriers have been in limbo because of funding issues and an ongoing review process, but Golden and the other experts contended there was no time to waste. During the two-hour boat tour they highlighted some of the neighborhoods that will be hardest hit if the project isn’t fast tracked, such as the South Street Seaport, Hoboken, Brooklyn Bridge Park, Red Hook, Ellis Island and Liberty Island.

Bowman said the city’s coastal areas have to deal with related environmental challenges: Rising sea levels and the attendant increased storm surges. By the end of the century, sea levels around the city will rise by 6 feet, according to Bowman.

Bob Yaro, president emeritus of the Regional Plan Association, added that a storm like Sandy was supposed to occur every 500 years, but now it’s estimated to happen every 24 years.

“This is something we have to plan now,” he said.

Golden said the surge barriers would play a key role in maintaining the city’s iconic waterfront, and, unlike other resiliency options, they would not ruin residents’ and visitors’ experiences.

“Imagine Ellis Island with a 25 or 30 foot wall around it,” he said as the boat passed.

Golden said his groups have brought the proposal to the city, state and federal levels and urged them to speed things. He said the governor’s office has given its approval to the plan.

A state spokesman couldn’t be reached for comment.

Seth Stein, spokesman for the mayor’s office, said the barrier is being evaluated by the Army Corps and that the administration has committed to several short-term post-Sandy projects, such as the installation of 4.2 million cubic yards of sand in Coney Island and the Rockaways.

“We are moving forward urgently with our $20 billion resiliency program in order to protect against a wide variety of climate risks,” he said.

A spokesman for the Army Corps of Engineers said it is reviewing the plan.

Brewer also called on the federal, state and city agencies to move forward with the barriers.

“The opposition should not be there,” she said.

City Councilwoman Margaret Chin, who represents lower Manhattan and sits on the Committee on Recovery and Resiliency, was on the tour and said she was impressed with the surge barrier.

“I will make sure the committee works more with them,” she said after the tour. “[The barrier] makes sense.”