Their job is patrolling the entire span of Chicago waterways -- 81 square miles of Lake Michigan and 27 miles of river.
"We have open-water swimmers, jet skiers, kayakers; we have those enjoying commercial tours, water taxis. We just have to make sure everybody is safe in the waters," said Lt. Erroll Davis, commanding officer, Chicago Helicopter and Marine Unit.
Davis is the new commanding officer. On the job only a few months, he has 25 years with the police department. It's a coveted assignment in the Chicago Police Department, but they also know how important the position is.
Davis said the hardest parts of the job are "uncertainty of the assignment until we get there, trying to make sure we do or search and rescue and recovery, make sure we can get everyone and recover those people."
Based at Dusble Harbor, the dispatch center monitors not only police, fire and marine radio, but Homeland Security locations and bridges. The exact number of personnel cannot be released for security reasons. But every officer in the marine unit is a trained diver.
The speed, along with night vision equipment and new detection sonars, will all help in future emergency calls, which number upwards of 25 a day during summer.
"A lot of times we get a call to search for weapons that might have been thrown in a crime," said Sgt. Ray Mazzola, Chicago Police Marine Unit.
The department agreed to show us how a typical dive in a weapons search might go down. Officer Jean McCarthy was tapped with the job. Before she goes in, every piece of equipment is checked.
"Diving is 85 percent at the surface," she said.
Divers are tethered to a communication device. In open water rescues, a wireless phone system is used that transmits through the water. Sometimes divers operate completely in the dark.