Rescue workers were searching for a British man, 24, who reportedly fell from a kayak into the reservoir of Rajjaprabha Dam in Surat Thani on Sunday morning.
Police said he fell into the water at about 9am while kayaking in the reservoir about 50-60 kilometres from the crest of the dam also known as Cheow Lan.
The accident happened at a spot where the water was about 50 metres deep.
Four scuba divers were trying to find him.
A local park director said the Briton and a friend stayed overnight at the Kraisorn Raft House. They took rented kayaks out early Sunday morning, but the friend later returned and told local officials that the missing Briton had fallen into the water about 500 metres from the raft house.
The spot was the deepest part of the reservoir, and divers lost visibility a few metres deep.
The park director said the tourist was not wearing a life vest.
