A man was seriously injured when he was thrown from a rubber ring pulled by a speedboat and struck by the vessel when it turned to save him.
The 49-year-old was riding the rubber doughnut in the Aberdovey Estuary, Gwynedd, on Saturday when he fell out.
He suffered serious cuts to his lower back and legs, the Maritime and Coastguard Agency said.
It is believed the injuries were caused when he was hit by the boat's propeller.
The boat's four passengers managed to get the man on board and the RNLI Aberdovey lifeboat towed the speedboat to YnysLas Beach, near Borth, Ceredigion where he was airlifted to Ysbyty Gwynedd hospital in Bangor by an RAF rescue helicopter.
A female member of the party on the boat was treated for shock, North Wales Police said.
Graham Warlow, Milford Haven Coastguard watch manager, said they had been alerted by North Wales Ambulance to the incident.
He said: "Thrill rides are inherently dangerous", adding that people need to carefully consider whether they had sufficient back-up and safety equipment before jumping in.
"We would like to thank the ambulance service, RNLI and RAF for their swift response and also take this opportunity to remind everyone that for the swiftest response to accidents at the seaside phone 999 and ask for the Coastguard."