One of the survivors called 911 on a cellphone, alerting authorities to their location seven miles east of Miami.
"Sadly, and tragically, we did find four females, adults, underneath the boat that had perished," said Coast Guard Cmdr. Darren Caprara.
The survivor found when Coast Guard officials flipped over the boat was suffering seizures, and he was taken by boat to a Miami Beach hospital, officials said. He was treated and released to federal law enforcement.
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 sent back 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.
The small white recreational boat with its center console missing was towed to dry land. It had been overloaded and lacked lifejackets, Caprara said.
It was not immediately clear whether those on the boat that capsized Wednesday were migrants fleeing their home countries. However, the circumstances made it seem likely that they were part of a global phenomenon of people taking desperate risks to escape poverty and instability, said David Abraham, who teaches immigration law at the University of Miami School of Law. In a case earlier this month, hundreds of migrants packed into smugglers' boats that capsized on their way from Africa to Europe.
"It should be no surprise to anyone so long as the disparity between the poorest place in the Western Hemisphere and the richest place in the hemisphere is so grave and the distances covered are considered worth the risk," Abraham 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, often through established smuggling networks that include islands in the Bahamas and Turks and Caicos.
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.
Officials in the Caribbean also have reported a jump in the number of arrests of Haitians making their way to Puerto Rico. An increasing number of Haitians have tried that route because if they can reach the U.S. territory without getting arrested, they can fly on to U.S. cities such as Miami, Boston or New York with just a driver's license or other identification, not a passport.
Since the 2010 earthquake, more and more Haitians also have fled for Brazil, which initially welcomed Haitians seeking asylum and later said it would issue a limited number of temporary work visas for Haitians.
"When people are desperate, they do desperate things. That's the problem. We're always concerned when people leave by boat and pay these smugglers," said Cheryl Little, executive director of the Miami-based advocacy center Americans for Immigrant Justice.
Haitians interviewed at an immigration detention center in Broward County routinely cite cholera, political unrest, dysfunctional law enforcement, ongoing displacement since the 2010 earthquake and, if they're women, vulnerability to sexual assaults in tent camps as reasons for fleeing Haiti in the hopes of receiving asylum in the U.S., Little said.
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.
The number of migrants who die while crossing the Florida Straits or disappear into South Florida's neighborhoods after successfully reaching shore is unknown.
Kay reported from Miami.