The floating corpse was reported to Chalong police station on the afternoon of November 23.
Police officers on a speedboat located the body the same day between the islands of Racha Noi and Racha Yai, about 25 km south of Phuket.
It was badly decayed: fish had eaten away skin from parts of the body, and only the skull remained of the head.
But the police think the corpse had been in the water no longer than 10 days.
“We are still unsure how long he has been dead because the body has been floating in the sea,” the Deputy Superintendent of Chalong Police Station, Pol Lt Col Jamroon Plaiduang, told The Phuket News.
The dead man was between 170 and 175 cm tall, and had a tattoo on the right forearm.
He appeared to have light skin, although the poor condition of the body made it hard to be sure.
“Although he is white-skinned and quite tall, we cannot assume if he is a foreigner or a Thai,” Col Jamroon said.
The body was clothed only in blue underwear with the brand name “Adder” when it was found.
Col Jamroon said the body was now in the morgue at Vachira Hospital while the police try to match the grisly find with any reports of people missing in the area.
Each police station in Phuket was checking with local resorts and hotels, but so far none had reported any missing guests, he said.
Anyone who may know about the dead man is asked to contact Pol Lt Wuttiwat Liangboonjinda at Chalong Police Station on 076 381 247 or 089 172 0305.