Saranuwat Nasok, a jet-ski rental operator at the southern end of the beach told police that the woman, identified as Ms Nina Ng, had rented a jet ski between 9:25am and 9:55am.
She was wearing a life jacket when she left the shore, he said.
However, by 9:50am, her jet-ski was found abandoned about one kilometre from the shore, with her belongings and life jacket left on board.
Concerned, Mr Saranuwat and others conducted a one-hour search but were unable to locate Ms Ng.
A boat captain operating nearby reported that at around 9:40am, he saw the tourist remove her life jacket and jump into the water.
Rescue workers from the Kusoldharm Phuket Foundation, along with dive teams and lifeguards, mobilised to assist in the search.
However, so far they have been unable to find her.
The incident has been officially recorded at Patong Police Station as efforts to locate Ms Ng continue, said Pol Lt. Wichyut Dampuek of the Patong Police