They said he had been traveling with nine other friends, and they found him drowning in front of Viking Cave, a famous coral diving site, news portal Thaiger reported.
They rushed him to a hospital where doctors provided him with CPR but were unable to save him.
The police are investigating the cause of his death and are working with the American embassy and his family.
Thailand has recently gained notoriety for a series of drowning accidents involving foreign tourists.
Last month a Russian tourist drowned off an island in Krabi while a German tourist died at Koh Samui‘s Chaweng Beach.