A ferry floating near the Rockaways is jarred by a mysterious explosion that knocks some 50 people into the water. Within minutes, New York Police Department harbor boats charge in with inflatable rafts and officers begin rescuing survivors with long hooks. Two helicopters swing out of the sky at speed, dropping dozens of water-activated lifesavers and a scuba diver into the channel. When the smoke clears, Emergency Service Unit officers storm the boat to determine if there is a terrorist threat.

Happily, it turns out that the “passengers” in this disaster in the Big Channel near Jamaica Bay are actually orange lifesavers and mannequins — not commuters desperately flailing in the drink. This is a training exercise for the police department’s Special Operations Division, a group that encompasses the most highly trained and specialized units of the police department: the harbor, aviation, and emergency service units.

The operation is no trifling matter: Dozens of officers participate and another group stands by in case something really does go wrong. The assistant chief who runs the division, Charles Kammardener, watches the exercise from the beach through binoculars.

“These are extremely dangerous exercises,” he says. “Our safety component is almost as big as the tactical component. But sometimes you throw them a curveball.”

This time, the unexpected circumstances focus on three mannequins that manage to “swim” to shore on a small island called Ruffle Bar. According to tags tied to their necks, they are badly injured and require emergency care. One mannequin has massive head trauma, burns on its face, back, and chest, as well as an amputated hand, the tag says. The only option, Chief Kammardener says, is a medical evacuation by helicopter.

Chief Kammardener and other top officials come up with these scenarios to train the city’s first responders. Often, they pluck their storylines from real life. When intelligence surfaced within the department about a possible threat against the subway tunnels, they did an exercise in the Brooklyn-Battery Tunnel. In the past, they’ve simulated a terrorist attack on the Statue of Liberty, chemical weapons on the subways, and a bus hostage situation, among other scenarios.

The question the leaders of this division deal with every day is: How do you train for anything? The answer, they say, is to get every officer prepared with a basic skill set and then plant unforeseen circumstances in the training.

“We don’t want them getting used to this,” a lieutenant with the Emergency Service Unit, Kenneth Beatty, says.

The exercises often take place away from Floyd Bennett Field in Brooklyn, where the units are based. In fact, as many real-life structures as possible are used. Units have trained at Lincoln Center, Broadway theaters, and in the subway system. A group trained on the Roosevelt Island Tram in the last few years, an exercise that proved helpful when the system stalled because of a mechanical malfunction in April. The police department led that operation, and Emergency Service Unit officers rescued every passenger without incident.

Not far from the waterfront where the ferry rescue operation took place, another Emergency Service Unit team was preparing for a different exercise. This time, they were storming a school where several students had taken over in a “Columbine-type incident.”

The units don’t know where they will encounter threats. They only know the location of the hostages, Lieutenant Beatty says.

The training never ends for these officers. An ESU officer spends scores of hours in the classroom and at locations around the country for specialized training every year. The unit does about 140 hours of Hazmat training alone to get certified for everything from disposal of hazardous chemicals to dealing with a weapon of mass destruction. On-site at Floyd Bennett Field are realistic training spaces, including an old Redbird train and a simulated disaster scene featuring overturned cars and rubble.

The daily bread for ESU officers is not, however, all the stuff of an action film. They also deal with gas line breaks, people pinned in cars, and serving warrants to dangerous criminals. The 400 or so officers are split up into eight trucks spread throughout the city. Their “Radio Emergency Patrol” trucks have Jaws of Life, giant air bags that can lift a train from the tracks, and an arsenal of weapons.

“You have to train for everything,” Chief Kammardener says. “And we have to do it while remaining completely operational in the city.”