Sombat Athiset, of the Katathani Phuket Beach Resort, said the incident took place off Kata Noi beach on December 10.
''The guest's leg was badly cut by a parasail speedboat and he was taken to Mission Hospital in Phuket City by our staff,'' Khun Sombat said.
The swimmer has been identified as Sergey Kuzennyy, 55. He was enjoying the water about 5pm when struck by a speedboat towing a parasail customer, Phuketwan has been told.
Photographs taken of Mr Kuzennyy's wounds show the distinctive slicing pattern of a propeller.
The incident comes with the future of jet-skis and parasail speedboats being hotly debated on Phuket.
Zoning introduced at Patong beach is supposed to protect swimmers from the dangerous vessels.
But many tourists do not know and are not being told that zones exist and continue to enter the water to swim in the jet-ski zones.
The Chief of Marine Office 5, Phuripat Theerakulpisut, said today he was aware of the speedboat hitting the swimmer at Kata Noi beach but said that zoning had yet to be introduced at that beach.
''My staff is still investigating,'' he said. It was not immediately clear whether parasail speedboats are permitted to operate at Kata Noi beach.
The swimmer dived under water about 15-20 metres off the beach and was struck by the speedboat as he surfaced, according to a witness.
''The speedboat was hauling a parasailer and could not stop even if they realised that they'd struck a swimmer,'' the witness said.
The speedboat operation is believed to be continuing without any contact between the driver and the man he struck.
News of the incident on Thursday appeared for the first time yesterday in the Thai media.
It was at a meeting in Phuket City on Thursday that Governor Nisit Jansomwong backed the trial zoning plan for jet-skis and speedboats for Patong beach. He added that any operator who broke the new rules would quickly lose their licence.
Marine Office 5 oversees the operation of jet-skis and speedboats on Phuket.
The neighboring provinces of Phang Nga and Krabi ban jet-skis. The machines were due to be phased out on Phuket over seven years but the present governor's predecessors tolerated compromises and allowed numbers to grow despite continuing reports of scams over insurance and extortionate claims for damage.
Parasails are considered to be even more dangerous than jet-skis, with take-offs and landings that continue on many Phuket beaches.
Russian tourist Nikolai Nefedov, 51, was struck by the propeller of a parasailing speedboat while swimming at Kata beach in December, 2011. One foot was badly cut. His leg was also hit.
According to a Phuketwan report at the time:
Near-misses are frequent at Kata, Karon and especially at Patong, where a new plan was announced at last week's meeting between government officials and Phuket honorary consuls to try to segregate the jet-skis and speedboats from the swimmers.