The U.S. Coast Guard responded around 1 a.m. after one of the survivors called 911 to report that the small vessel had capsized seven miles east of Miami.
"Sadly, and tragically, we did find four females, adults, underneath the boat that had perished," said Cmdr. Darren Caprara.
Most of the survivors were found clinging to the hull, and another was found in an air pocket beneath the boat, officials said.
One survivor was taken by boat to Miami Beach, where he was treated at a hospital and released to federal law enforcement, Barney said.
The rest of the survivors were in good condition and were taken into custody aboard a Coast Guard vessel while authorities investigated whether they were part of a human smuggling operation. It was not immediately clear whether they would be brought to the U.S. or repatriated to their home countries.
"Well, obviously, 15 people on a boat, transiting in the middle of the night with no life jackets is a very, very unsafe condition," Caprara said.
Caprara said that authorities were working to confirm that the people on the boat were Haitian and Jamaican.
"That's still a lengthy process that involves contacting other countries and doing some investigatory research," Caprara said.
Images of the vessel show a small white recreational boat with its center console missing. It was overloaded and lacked lifejackets, Caprara said.
Migrants from Haiti, Cuba and other Caribbean countries routinely attempt to illegally enter the U.S. by reaching Florida's coast in overloaded or unseaworthy vessels.
In the fiscal year that ended Sept. 30, the Coast Guard picked up 508 Haitians and 1,357 Cubans at sea. Since the new fiscal year began Oct. 1, the Coast Guard has reported picking up 93 Haitians and 117 Cubans.
The number of migrants who die in the crossing or disappear into the community after successfully reaching shore is unknown.
Cubans who arrive in the U.S. are generally allowed to stay under the "wet foot, dry foot" policy, while those stopped at sea are usually returned home. Other immigrants who make it to land don't receive the same treatment.
Kay reported from Miami.