The Caucasian was found unconscious on the beach the following morning but at that time no ID was found on him. He had a wound on his head, inflicted by a long piece of wood found nearby.
He was taken to hospital and later identified as Jonathan Mark Lemley, of Washington State in the USA. He is still unconscious.
Kitti Thiradeewattananuksan, 18, surrendered last night, coming into Karon Police Station accompanied by the local Kamnan, Winai Chidchiew.
He explained that on Friday night he and some relatives were pulling up their fishing nets from Nong Harn when they noticed a foreigner, who appeared to be drunk, driving onto the beach nearby.
The foreigner got stuck in the sand so Kitti and his relatives went to help him get out of the sand. The foreigner, presumably leaping to the conclusion that he was about to be robbed, punched Kitti, knocking him down, then grabbed a piece of wood and tried to hit Kitti with it.
Kitti explained that he kicked the foreigner’s knee so that he, too, fell. Kitti grabbed the piece of wood and hit the foreigner once on the head before walking away.
It was only when he saw news reports later that Kitti became aware that he had done the foreigner some real damage, and decided to turn himself in.
Mr Lemley is in Vachira Phuket Hospital where he appears to be recovering, though he is not yet fully conscious, said Karon Police superintendent Col Nattapakin Khawnchaiyaperng.
When he does recover he is likely to face questioning not only about the beating but also about hashish, cocaine, methamphetamine pills and crystal meth allegedly found in his pockets.