Petch Pengjam, 40, was drinking on the beach with five friends and decided to go for a swim, wearing jeans.
Komsan Naithong, 23 one of the friends, said the six were relaxing at the beach on their day off.
When Mr Petch went into the water and was knocked down, his friends shouted for help from lifeguards.
After 10 minutes a lifeguard located Mr Petch, who by then was unconscious.
On the beach the lifeguard gave CPR, and Mr Petch was taken to Thalang hospital. Doctors spent an hour trying to revive him, but without success.