The man was riding the doughnut in the Aberdovey Estuary, Dyfed, Wales, on Saturday when he fell out.
He was hit when the boat returned to rescue him and suffered serious gashes to his lower back and legs, the Maritime and Coastguard Agency said.
The four occupants of the boat managed to get the man on board and the RNLI towed them back to Ynys Las Beach, near Borth. He received medical attention before being airlifted to hospital by the RAF.
Milford Haven Coastguard watch manager Graham Warlow said: "Thrill rides are inherently dangerous and people need to carefully consider whether their skipper has the necessary expertise and safety equipment before they jump in.
"We'd like to thank the ambulance service, RNLI and RAF for their swift response but also take this opportunity to remind everyone that for the swiftest response to accidents at the seaside phone 999 and ask for the coastguard."