Rescuers carried Lin Zhang, 23, to the surface where she failed to respond to resuscitation then to the nearest hospital at Thai Muang, in Phang Nga province, not far from the popular holiday dive centre of Khao Lak.
The young woman, diving with friends from the vessel Peter Pan with the Similan Seven Sea Club, was found by divers from Khao Lak Scuba Adventures about 2.30pm off Bon island, according to friends.
Bon island is part of the Similans national park, which is usually popular with divers at this time of the year.
The diver was floating face-down and her mouthpiece was not in place. She appeared to have vomited underwater.
Her dive watch last recorded a final depth of 11.2 metres. Much of the dive was spent at nine metres.
Others on the boat said she disappeared after having lunch, on the third dive of the day.
She had previously made 20 dives, including three on Wednesday.
A speedboat carried Ms Zhang back to the Andaman coast and she was taken to Thai Muang Hospital. In late afternoon, examining doctors said she had been dead for approximately six hours.
She had a tattoo of six small birds on her shoulder and appeared to have been diving dressed in a piece of headgear that had two yellow tiger's ears and a yellow and black tiger tail.
An autopsy is likely to be considered to determine the cause of her death.