Police investigators are now trying to determine whether the man fell or was pushed and why he was not wearing a life jacket, as tourists are required by law to do.
The vessel, carrying about 30 passengers, was travelling to Koh Phangan from the smaller island of Koh Tao, where the vicious murder of British tourists Hannah Witheridge and David Miller has captured international attention since September 15.
Police from Phangan are investigating the circumstances of the manner in which the crew of the vessel, the Seatran Discovery, turned around and pulled the man from the sea. Life jackets are compulsory.
Colonel Prachoom Rungtong, Superintendent of Phangan Police Station, said that the man, aged 26, was spotted leaning over the rear of the vessel, photographing the propeller, before he fell.
''It's unclear whether his mental state has any bearing on what happened,'' the colonel said. ''We will be asking the captain why there was no safety vest and why he wasn't pulled alive from the sea.''
The investigation into the Belgian man's death comes amid growing concern about the safety and security of tourists in Thailand, following the murders on Koh Tao.
A suggestion that tourists wear wristbands, made recently by the Tourism and Sports minister, probably would not have helped. A life vest, however, might have saved his life.
Phuketwan has been given the man's name but will not identify him until we can be sure his family has been notified.
His body was taken to Koh Phangan Hospital.