The body was found off Koh Naha, off the east coast of Phuket, said Thalang Police Investigation chief Lt Col Pathiwat Yodkwan, who was notified of the discovery of the body at about 11am.
The body had been discovered by the driver and staff of a tour boat taking tourists to visit Koh Khai.
The body was brought back to Thiam beach.
Dressed in a brown shirt and black underwear, the man, who was about 170cm tall, had his face beaten and teeth broken. Police found no identification documents on the body, which was taken to Thalang Hospital for a post-mortem examination.
Investigators initially believed the dead man might have been a boat crew member who was assaulted and then thrown into the sea. The investigation is continuing.
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