Every September, Americans take time on the anniversary of 9/11 to share their memories of that fateful day, recall how they first learned the news and pay tribute to the heroes who answered the unexpected call of duty.

As a nation we remember the first responders who ran into the World Trade Center and the Pentagon. We reflect on the sacrifice of the passengers aboard Flight 93 who charged the cockpit in the sky above Pennsylvania. We honor the many who enlisted to serve our country in the wake of the attacks to bring justice to our attackers.

And while we may find ourselves divided on many issues nearly two decades later, we are bound forever by the words “Never forget.”

On this year’s anniversary, one relatively unsung 9/11 story deserves retelling. It is the story of ordinary citizens who risked their lives to save more than a half-million people trapped at the southern tip of lower Manhattan — a rescue effort that would become the largest water evacuation in recorded history.

After the first of the Twin Towers collapsed, there was widespread confusion and panic. We were suddenly at war. We didn’t know what was coming next. Tunnels, bridges and highways leading out of the island of Manhattan were shut down.

There had never been a plan for how to conduct a mass evacuation from the most populous city in America — that was unthinkable. For the first time in more than 100 years, the only way on or off the island was by boat.

And then unexpected saviors came to the rescue: American mariners.

It started slowly as random boats and ferries already in the water voluntarily turned around and started loading people, as many as possible. It was instinct at work — patriotic Americans not wanting to leave each other stranded and vulnerable to whatever was coming next.

But the initial fleet of boats could fit only so many. The US Coast Guard, our nation’s maritime first responders, knew it had to organize. Officers got on the radio and called out to all nearby mariners: “Come help!”

And come they did. Tugs, party boats, water taxis — if it could float, it was steaming toward lower Manhattan, despite the risk of additional terror attacks and exposure to smoke and debris.

These mariners simply were not going to leave anyone behind. They loaded up as many individuals as could fit, dropped them off and sailed right back to the island, over and over again, all day and into the night.

In all, nearly 500,000 people were evacuated that day, more than the 339,000 rescued at Dunkirk. Some 150 different vessels, crewed by more than 800 American mariners, improvised and successfully executed this extraordinary feat of bravery.

Long-standing maritime traditions — safety, commitment, courage — guided these heroes.

Men like Vincent Ardolino of Brooklyn, captain of the Amberjack V, who passed away last year but whose stirring words can still be heard in the 2011 Tom Hanks-narrated documentary “Boatlift: An Untold Tale of 9/11 Resilience” as he recalls what could’ve been his final farewell to his wife: “I’ve got to go do something … I’m going to take the Amberjack up into the city and help … I have to do what I have to do … Even if I rescue one person, that’s one person less that will suffer or die.”

America’s maritime industry is accustomed to working in quiet anonymity to protect the nation and keep the economy moving. Yet, to those familiar with us, the actions of men like Vincent Ardolino on Sept. 11, 2011, come as no surprise.

In times of war and in times of peace, the American mariner will step up to serve without thinking twice.

And on that awful day, their aid — like that of so many brave first-responders — proved indispensable.

Fact is, this nation is blessed with many heroes willing to rush to help their neighbor, even at risk to themselves and without any desire to be singled out for their heroism.

Our mariners demonstrated that with crystal clarity on 9/11. As a maritime nation, we should count ourselves fortunate.

James Henry is chairman and president of the Transportation Institute.