Thalang Police were notified the incident at 5:25pm.
The tourist, identified by her passport as French national Line Claudine Robinet, 57, suffered a broken left arm, a broken hip and injuries to her left knee, police confirmed.
She was rushed back to Thalang Hospital on Phuket, and has since been transferred to Bangkok Hospital Phuket.
According to the report of the incident issued to the Thalang Police, tour guide Somchai Kongkum, 57, said he that at 4:05pm he called all the tourists in his group to start boarding the boat to return to Phuket.
However, Ms Robinet wanted to go for a parasail, and Mr Somchai agreed. While she was flying through the air, he was loading his tour group into the boat for the return trip.
While he was guiding the tourists back into the boat, Ms Robinet landed hard on the roof of the boat, he said.
“She then fell hard into the back of the boat, while the parasail and ropes fell into the water,” he added.
Kusoldharm Foundation rescue workers met the boat when it returned to Phuket and administered emergency first aid to Ms Robinet before speeding her to Thalang Hospital.
Thalang Police confirmed that they had recorded the accident and were continuing their investigation into the incident.