Photo: Stephen Schafer.
The Martin County Sheriff's Office said a lifeguard first saw the 38-year-old Schafer in distress Wednesday about 500 yards from shore, near the town of Stuart, about 100 miles north of Miami.
The lifeguard paddled out and found several sharks encircling Schafer, put him on his rescue board and paddled back to shore. Paramedics took Schafer to a hospital, where he later died.
While swimmers and surfers often receive minor bites in Florida waters, officials say fatal shark attacks are extremely rare.
Research scientist Grant Gilmore told Scripps Treasure Coast Newspapers the attackers were likely juvenile great white sharks.
A childhood friend of Schafer's said it was unusual for him to be in the water alone. Teague Taylor said, "it's hard to believe...(he) would make that one mistake."
Despite the deadly attack, beaches in the area remained open Thursday.