The alarm was raised after one person was found clinging to the vessel by a "good Samaritan" about 45 miles east of Fort Pierce Inlet.

The survivor - who was taken to the local hospital to be treated for dehydration and sun exposure - said he and 39 other people had left Bimini in the Bahamas on Saturday night.

He said the boat then capsized in bad weather.

The man also said no one was wearing a life jacket.

Coastguard crews on the water and in the air are looking for the missing and have so far searched an area of more than 1,300 square miles.

The coastguard later posted a photo on Twitter of the survivor who was discovered on the vessel, with much of it submerged.

In the update, it said: "@USCG crews are still searching. The good Sam notified USCG Sector Miami watchstanders, Tuesday, at approx 8am after rescuing a man on a capsized vessel. Multiple cutters & aircraft are searching from Bimini, Bahamas to FortPierce Inlet."

The islands of the Bahamas have been frequently used as a stepping stone to reach Florida and the United States, and there have been thousands of deaths over the years.

Last Friday, the coastguard rescued 32 people from a suspected migrant boat that had capsized about five miles west of Bimini.

On the same day, 88 Haitians were intercepted by the coastguard and Royal Bahamas Defence Force onboard an overloaded sail freighter near Great Inagua in the Bahamas.

"Coast Guard crews continue to have a presence patrolling the waters around Haiti, Dominican Republic, Cuba, Puerto Rico, and Bahamas, to ensure the safety of life at sea," Capt. Benjamin Golightly said at the time.

"Navigating the Florida Straits, Windward and Mona Passages in overloaded and less than seaworthy passages is extremely dangerous and can result in loss of life.

"The Coast Guard and our partner agencies will continue to stop these voyages. You will be interdicted and should expect to be returned and repatriated to your country of origin."