The Chinese embassy in Bangkok said in a statement that it had requested the Thai government make all-out rescue efforts and that it had sent a team to Phuket to help.
A navy deputy commander, Charoenpol Kumrasri, told a news conference Friday morning that they hope survivors can be found in the boat, which sits about 30 meters (98 feet) below the surface.
Another 39 people were saved when a second boat, carrying mostly European passengers, overturned at about the same time.
Phuket's governor has declared the area a disaster zone.
A boat carrying dozens of Chinese tourists overturned Thursday in rough seas off southern Thailand and 49 passengers were unaccounted for, an official said.
At least 12 of the injured were hospitalised.
The Royal Thai Navy said "quite a few bodies" were found inside the sunken boat.
All boat passengers were Chinese tourists, officials said.
"Helicopters will search for those who may have floated out".
Fifty-eight tourists on board a diving boat and yacht remain missing after the two capsized in rough seas off the popular Phuket Island in southern Thailand yesterday.
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Thailand is already making worldwide headlines after 12 boys and their junior soccer coach were found alive this week after becoming trapped inside a cave in the country's north.
A total of 48 passengers were rescued by a fishing trawler on Thursday, while another survivor was pulled from the water several miles from the boat, along with several bodies, according to AFP.
The second boat, the Serenita, had almost 40 passengers on board, Sophakarn said.
Some cried while others appeared dazed as they walked around still wearing their life vests.
Several other vessels hit trouble late Thursday.
A Chinese consular official has arrived at the operations centre in Phuket to monitor the rescue effort.
But as details dripped through overnight, governor Noraphat said two passengers were still unaccounted for from another stricken ship.
Phuket draws hordes of overseas visitors including Western sun-seekers and huge numbers of Chinese tourists who make up almost a third of the 35 million people expected to visit Thailand this year.
Thailand is in the middle of its rainy season, which usually runs from May to October, and often generates high winds and flash storms in coastal areas, especially on its west coast on the Indian Ocean, where Phuket is located.