An American teacher was charged with negligent homicide after his son's friend drowned during a beach outing in Thailand.

The American national, 45, whose name was witheld by police, went to the Ao Thung Sang bay in Chumphon province along with his wife, son, and the youngster's friends, on March 5.

As they rested on the shore, the man's son and Wiwat Phuthong, 8, waded out into the surf to play in the sea, bringing along a fish-shaped rubber inflatable.

The pair were enjoying a dip in the water around 10 to 16 metres from the sand when a large wave slammed into them.

The American's son managed to cling onto the rubber buoy and was rescued after his cries for help were heard by a nearby Burmese worker. However, Wiwat was swept to sea.

Pathio district police said they received a report about the missing boy at 6 PM and coordinated with rescue teams and divers to search for him.

More than 100 officers and rescuers combed the sea for hours but failed to find Wiwat. The mission was temporarily halted at 10 PM.

The search resumed the following afternoon, as strong waves hampered efforts in the morning.

Wiwat's remains were found by a local fisherman around one kilometre from where he disappeared. His body was sent to the Pathio Hospital for a post-mortem examination.

Police found that the American teacher was a local Christian church worker who brought his family and son's friends to relax on the beach.

He has been charged with negligence resulting in death.