The 87-year-old man rented a beach lounge about noon near Loma Park, an attendant said.
About 3.15pm, the man was spotted some distance off shore, not moving. A jet-ski was sent to bring him in.
The man was dead when he reached the beach, lifeguards said. Police were in the process of telling Swiss Embassy officials in Bangkok.
It's believed he may have suffered a cramp and drowned.
His body was taken to Patong Hospital. On Sunday, the body of Dutchman Stephan Buczynski, 26, was found at Patong beach.
An autopsy is being undertaken in Bangkok to confirm a Patong doctor's opinion that he drowned. Mr Buczynski also had a four-centimetre cut to the back of his head.