The man ran from the passenger compartment of the Seatran ferry when it was about seven kilometres off the coast, said Pol Capt Thepsingha Thepluen, a district police investigator.
The man's mother said she had been taking her 20-year-old son from the island to meet a doctor at Suansaranrom Psychiatric Hospital in Phunphin district of Surat Thani.
She said she tried in vain to grab her son to stop him from jumping. Passengers and crew threw life vests into the sea before he went under, she added.
Rescue workers from the Kuson Sattha Surat Thani foundation in Muang district were dispatched to the area but strong waves in the Gulf of Thailand impeded attempts to locate the man, the officer said.
The man's mother said her son had experienced mental problems and had triedf to commit suicide before.