The incident occurred about 5:30 p.m. EDT off Key Biscayne, The Miami Herald reported.
An unidentified person who was on the catamaran told the newspaper a group of friends had hired the boat for the day and it began to take on water for unknown reasons while returning from a sand bar. Other boaters helped the Coast Guard and local authorities rescue the passengers within minutes, the Herald said.
No serious injuries were reported.
"It was no big deal," one unidentified party-goer with a green children's inner tube told the Herald after being shuttled to shore.
Florida Fish and Wildlife Conservation Commission spokesman Jorge Pino said the boat may have been an illegal charter that charged $20 per person and didn't appear to have a licensed captain.
Pino described the catamaran as a homemade vessel assembled from "many different things."