Three teenagers were thrown from their boat which was left "dangerously" out of control off the Dorset coast, coastguards have said.
A number of 999 calls were made reporting the youths in the water off Studland, outside Poole Harbour.
A passing yacht managed to rescue one of the teenagers while the other two managed to swim ashore. One was treated for hypothermia and cuts and bruises.
An RNLI lifeboat stopped the out-of-control boat and towed it to shore.
The teenagers were given safety advice about using kill cords which stop the engine in the event of someone falling out of the boat.
Bruce Lack, watch manager at Portland Coastguard, said: "A kill cord is an essential piece of safety equipment.
"It is useless unless worn and the recommended use is to wear around the thigh of the driver.
"Older outboard engines can be retro-fitted with a kill cord very simply and easily.
"It is recommended to fit one and use it. It may help save your life, in an emergency."