COASTGUARDS today warned that four people could have died in the water when their speedboat sank.

Lifeboat crew plucked two men, a woman and a teenage boy from the sea after their Fletcher Arrowflight craft rapidly sank half a mile from Lyme Regis.

The Portland Coastguard helicopter airlifted the casualties to the Dorset County Hospital in Dorchester amid fears they were suffering from hypothermia.

Coastguards today praised the helicopter crew and Lyme Regis inshore lifeboat for their quick rescue.

Ros Evans, watch manager at Portland Coastguard, said: "Without a doubt due to the very cold temperatures of the waters offshore at this time of year, about nine degrees Celsius, this incident could have had a tragic outcome had they not been pulled from the water in double quick time by the lifeboat crew.

"Fortunately all of the casualties were wearing lifejackets."

The casualties - a couple in their 30s and a father and his 13-year-old son from Taunton - were treated for mild hypothermia and released from hospital.

The drama unfolded at 7.30pm on Saturday night when the passengers on the 14-foot long speedboat raised the alarm by calling 999 on a mobile phone.

They said that their boat was sinking quickly and that they needed rapid assistance.

The coastguard operator was taking further details when the caller said that they were going under and the call was cut off.

The coastguard helicopter scrambled and the Lyme Regis inshore lifeboat launched immediately. The lifeboat arrived 45 seconds after it was launched and crew found the four swimming around their sinking boat. They plucked the casualties from the water and helmsman David Street quickly returned to harbour amid fears the four were at various stages of hypothermia.

The coastguard helicopter landed on the beach as members of the lifeboat station crew carried the casualties ashore.

It airlifted the four to hospital where they received treatment.

The boat, which was submerged but for a pocket of air keeping a small section of the bows afloat, was towed back to harbour.

The hole in the boat was caused by a part of its trailer.

Coastguards warned people to be prepared before setting out.

Ms Evans said: "Please check the weather forecast before you put to sea and make sure that conditions are suitable for the type of vessel you are using."