Zhang Zhang, 27, had gone with four friends in a tour group of about 40 to the popular day-trip diving and snorkelling island, south of Phuket.
Officers from Chalong Police Station said that a tour guide had advised the group not to go swimming too soon after lunch.
Zhang Zhang's friends noticed him missing and began a search along Ao Tawan Tok, a beautiful bay where the sea is usually tranquil.
His body was found floating about an hour after the search began. Efforts to revive him failed.
His body was brought back by speedboat and is now at Vachira Phuket Hospital in Phuket City.
Mr Zhang and his friends arrived on Phuket on April 29. They had been staying in Patong and were due to fly home today.
Officials at the Chinese consulate in Songkhla, which oversees the welfare of the increasing number of Chinese tourists visiting Phuket, says many are not familiar with the sea.
''Most do not understand even the rudimentary skills involved in using a snorkel and mask,'' an official said.
''The more they can be briefed about dangers in the water and how to survive, the better.''
Chinese visitors to Thailand are reported to have almost doubled year-on-year in the first three months of 2013, with four million expected for the whole year.
Many of those will try Phuket and be experiencing a beach destination for the first time.