Seven of eight passengers were rescued from a sunken charter boat near Islamorada on Sunday, the U.S. Coast Guard said.
The eighth passenger, a 36-year-old woman, was pronounced dead.
According to the Coast Guard, the 30-foot commercial diving boat called Get Wet started taking on water and then capsized at Molasses Reef. Two passengers became trapped within the hull.
The incident was reported to the Coast Guard by a good Samaritan, who also helped rescue the people.
Authorities responded and a commercial salvage company was contacted for the capsized vessel.
"The Coast Guard is thankful for good Samaritans on the water who maintain a watchful eye and stand ready to assist boaters in distress," Coast Guard Capt. Pat DeQuattro said in an email statement. "Our thoughts are with the family and friends of the victims of this tragic occurrence."
No other details were immediately available.