The charter aircraft, which belonged to Chalk's Ocean Airways, crashed in shallow shark-infested waters at the entrance to Biscayne Bay, which separates the cities of Miami and Miami Beach soon after taking off on Monday afternoon.
A flotilla of civilian, police and Coast Guard crafts and several helicopters sped to the crash site, but were unable to find any survivors.
The airplane, under several feet of water, was upside-down and visible from the air, as Coast Guard divers plunged into the ocean seeking survivors.
The plane was a 17-passenger Grumman G-73T Turbine Mallard, a twin-engine amphibious aircraft capable of operating from water as well as land-based airports, according to the company website.
Roger Nair, director of operations for Chalks Ocean Airways, said: "The flight departed the sea plane base at Watson Island on its way to the island of Bimini in Bahamas."
While the company and the Miami Beach firefighters first said there were 20 passengers, the Coast Guard said that 19 people were aboard. Coast Guard officials earlier said three infants were among the passengers.
"It went right over my head, when I looked up it was on fire. I really thought it would land on us"
One witness, who identified herself only as Terry W, was surfing close to the rocks with a group of friends at the time of the crash.
"It went right over my head, when I looked up it was on fire. It was about 20ft above the water. I really thought it would land on us."