The boy and his mates swam out from a small beach at Koh Sireh, east of Phuket City, about 1.30pm.
However, Kerttisak Tongkeaw complained of feeling tired in the water. When the group of boys reached the beach, near a floating speedboat pier, Kerttisak was nowhere to be seen.
''We went looking for him,'' one boy later told the Kusoldharm Phuket Foundation rescuers.
''But we couldn't find him, so we let our parents know that he was missing.''
Kusoldharm rescuers recovered the boy's body and carried him back to the floating pier.
Lifeguards patrol Phuket's well-known west coast beaches but there are no patrols or ''safe to swim'' flags on the east coast beaches, which are not considered dangerous.