Picower's wife, Barbara, discovered his body and pulled it from the water with help from a housekeeper, authorities said. He was pronounced dead at Good Samaritan Medical Center at about 1:30 p.m.
Palm Beach police are investigating the death as a drowning but have not ruled out anything on the cause of death.
Picower suffered from Parkinson's disease and had "heart-related issues," said family attorney William D. Zabel. He described Picower's health as "poor."
Picower's body showed no visible injuries, said Joseph Sekula, spokesman for the Palm Beach Fire Department.
"There wasn't anything noted as far as trauma or anything to the body," he said, adding that "it did appear that he was swimming because he was wearing swimming trunks."
Detectives were still at the home more than six hours after the initial 911 call. The iron gate to his long driveway was open and several Palm Beach police cars were parked near the mansion. The home and property are worth more than $33 million, according to the county property appraiser's records.
Picower had been accused by jilted investors of being the biggest beneficiary of Madoff's schemes.
In a lawsuit to recover Madoff's assets, trustee Irving Picard demanded that Picower return more than $7 billion in bogus profits. In an e-mailed statement Sunday, Picard said only that "litigation will continue."
Madoff's attorney, Ira Sorkin, did not respond to a request for comment.