Monroe County Sheriff's officials say 70-year-old James Gogan was found floating in the water near a boat he had been working on.
Gogan's girlfriend told deputies she and a neighbor found him in the water Thursday afternoon.
Deputies say Gogan had been working on the boat near his home in Marathon earlier Thursday.
According to Gogan's girlfriend, he had some sort of heart condition that he planned to get check out next month during a trip to New Jersey.
Foul play is not suspected. No further details were immediately available.