A man who had a leg cut off and lost part of the other when he was struck by a propeller blade on Southampton Water was trying to climb on-board a boat.
Police said the 40-year-old was a passenger on a jet ski and part of a group of seven adults taking part in water sports on Saturday afternoon.
They were returning to Hythe Marina on the jet ski and a boat at the time.
The man had entered the water and was attempting to board the speedboat when his legs got sucked into the propeller.
A spokesman for Hampshire Constabulary said: "The engines were running and the propellers were turning. It would appear the person's legs were drawn under by the propeller and severe injuries were incurred.
"In fact one leg, it is believed, has been severed completely and the other
leg has been severely damaged."
'Rare' injuries
The coastguard helicopter was scrambled to the scene shortly after 1600 BST after a witness reported seeing the man "in sheer agony", a spokeswoman for the Maritime and Coastguard Agency said.
The injured man, from Southampton, was airlifted to Southampton General Hospital. He lost his right leg and part of his left leg.
Colin Griffiths, watch manager at Solent Maritime Rescue Co-ordination Centre, said: "The scale of the injuries to this person is rare in our experience. Our thoughts are with him and his family at this time."
Hampshire Police were at the scene and the Marine Accident Investigation Branch launched a full investigation into the incident.
Southampton Water - a 28km long waterway - is one of Europe's busiest shipping routes.