In the murky waters of New York Harbor near Bay Ridge, Brooklyn, a boat carrying seven passengers headed home last week after catching Fourth of July fireworks. But the boat got stuck, and a team of city firefighters turned up to answer its distress call.

"Their props got wrapped in a line, and they got stuck and were unable to move. They were dead in the water," said Lieutenant Chris Childs with FDNY Marine 8. "So we had to board their vessel and cut these four-inch diameter ropes, there were about five of them off their boat, to free them, and we set their anchor and waited for a commercial tow."

Childs, a lieutenant from Staten Island's Marine 8, says his unit responds to about 300 similar calls from May 1 through November 1. 

Marine 8 is part of the Rapid Response Craft, or RRC, division of the FDNY's Marine Operations unit. Here, a fire officer and two firefighters are assigned to small patrol boats that respond to all sorts of emergencies around the five boroughs.

"When something goes wrong, it goes wrong very fast," said Brett Hansen of the FDNY.

There are three rapid response boats that work seasonally, along with three others that patrol the city's waters year round. 

The rapid response boats share a distress channel with other first responders, and depending on which unit is closer, a call could be answered by the NYPD, the Coast Guard or the FDNY. 

But the many kayakers, boaters, and swimmers in need are often surprised to see the Fire Department out on the water to help.

"A lot of times, they don't know what we do. They're very amazed that the Fire Department is showing up in the middle of nowhere," said FDNY Battalion Chief Joseph Abbamonte.

The boats are responsible for patrolling 560 miles of coastline in the city and in sections of New Jersey.

The boats can travel up to 50 miles an hour and come with radar tracking to warn of objects in the water, and estimate precise locations for where a response is needed. 

But don't let these calm waters fool you. With big increases in recreational boating and commercial transportation, the FDNY says the marine unit is busier now than ever before.