Ting Hong, 29, was pronounced dead after medical staff at the Racha Resort Hotel on Racha Yai Island tried in vain to revive her after she was taken from the water.
Her body was later taken from the island to Vachira Phuket Hospital.
Pol Lt Wutthiwat Liengboonchinda, an officer on duty at Chalong police station, said Ting and about 30 other tourists had taken a one-day boat trip from Chalong Bay in Muang district to go snorkelling off Racha Yai Island.
Tour guides and staff showed the visitors how to use snorkels and instructed them to wear life jackets, the police said.
Witnesses told police that the Chinese woman was later found lying unconscious with her face down in the sea. She was wearing her life jacket and snorkel at the time.
She was rushed for first aid at the hotel where she died.