A quick-thinking fisherman who fell overboard managed to cling on to his nets for five miles and was dragged behind his boat back to shore.
Nathan Rogers, who is in his late 30s, was "amazingly" lucky to survive the ordeal off Cornwall, the RNLI said.
He had been adjusting the nets when he fell in but the boat kept powering forward on autopilot until it crashed into Newlyn's South Pier on Thursday.
A lifeboat brought him to shore and he was treated for severe hypothermia.
Penlee Lifeboat coxswain Patch Harvey said a young boy raised the alarm early on Thursday afternoon when he spotted the "very well known and popular fisherman" in the water, about 90ft (27m) from his vessel.
Eleven-year-old Louis Matisse Nicholls said: "I was beach cleaning and I saw a man fall off his boat.
"I just saw a big splash and I saw his head bobbing up and down, and I just saw the boat crash right into the harbour. I told my dad to phone 999."
When the lifeboat arrived Mr Rogers was being held up in the water by the skipper of another fishing boat.
"He was very, very cold," he said, "We are just amazed he managed to hold on for so long."
He added: "He was moving his hydraulic hauler from one side of the boat to the other and fell over with it. Luckily the hauler was connected to the boat with hydraulic hoses and they managed to stay intact all the way in.
"If they had snapped he wouldn't be here now."
It is believed the boat was travelling at speeds of up to six knots, or about 7mph (11kmph).
Mr Rogers was now recovering at home, the RNLI said.