The woman, Jang Sia Dan, boarded the tour boat King Poseidon 23 in Phuket this morning with 19 other tourists to enjoy a day trip to the islands in Phang Nga Bay.
Ms Jang was injured while the boat was moving to moor at Pileh Bay at Phi Phi Island at about 9am, the TAC officer said.
She was getting off the back of the boat to snorkel while the captain was reversing the boat to moor. She suffered severe wounds to both legs.
Ms Jang was rushed to Koh Phi Phi Hospital for immediate treatment then sped back to Phuket to receive more specialised treatment at Vachira Phuket Hospital in Phuket Town, the officer confirmed.
The tour group was led by a 37-year-old Chinese tour guide. Thai skipper Chatchai Khacharoen, 26, drove the boat and Yingyong Khongkhamsuk, 26, was a boat staffer, reported the Bangkok Post. (See story here.)
Mr Chatchai told police that he did not see Ms Jang getting off the boat, so he continued to bring the boat to moor.
But after other tourists shouted, he immediately stopped and saw the injured tourist shouting for help.
Capt Chatchai and his assistant pulled her back on the boat and promptly had her taken to the hospital, he said.
Mr Chatchai, his assistant and the tour guide were held in police custody at Phi Phi Police Station. They were initially charged with reckless collusion causing serious injuries to another person, noted the Bangkok Post report.
Tourist numbers from China have declined in recent months after the Phoenix tour boat disaster that killed 47 Chinese on July 5. Chinese arrivals fell 6.7% month-on-month in August and were down a further 25.3% in September and 0.2% in October.
Authorities have been taking steps to improve the image of the country among Chinese tourists, including a waiver of visa-on-arrival fees and improved safety measures for land, air and water transport, the Bangkok Post noted.