Rescue crews found a man in Biscayne Bay after the boat he was in started taking on water.
Another man on board made it to shore to call for help and a third was rescued by the Coast Guard earlier in the evening, said Miami Fire Rescue Captain Ignatius Carroll.
The call for help came in just after 10 p. m. off the Julia Tuttle Causeway between the two bridges separating the mainland from Miami Beach.
It was not immediately clear what caused the boat to take on water, but the man was in the water about an hour, Fire Rescue said.
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He was in cardiac arrest when he was found and transported to the hospital, Carroll said.
Carroll said the man who was rescued by the Coast Guard earlier in the evening was taken to the hospital for treatment.
Agencies involved in the search include Miami Fire, Miami Beach Fire, Florida Fish and Wildlife Commission and Miami-Dade police.