The upturned hull of the capsized boat

Ten day-trippers, who had hired the 20-feet-long boat from Wroxham, were out on the River Bure on Saturday night when it hit a bank and flipped over.

Eight of them scrambled to safety but the woman, a 42-year-old from Middlesex and her 49-year-old sister from Suffolk, were trapped inside an air-pocket for about an hour.

The sister, who have not been named, were airlifted to the Norfolk and Norwich University Hospital after being freed.

One died and the other is being treated for hypothermia and severe shock.

They told me that there were two women trapped and that people had tried to dive under the boat to reach them Flight Lieutenant Chris Marriot

Coastguards said the cruiser, which was on hire from Barnes Brinkcraft, overturned shortly before 1800 BST on the River Bure at Horning.

RAF rescue helicopter crewman Flight Lieutenant Chris Marriot, 25, told of his desperate attempts to rescue the sisters who were trapped up to their necks in water.

He was one of five rescuers on an RAF search and rescue helicopter sent to the scene from its base in Wattisham, Suffolk.

He was winched down to the river bank and waded through five feet of water before climbing on to the hull of the upturned boat, cutting a hole in the hull and pulling the women out.

"It was relatively easy wading out to the boat because the water was only five feet deep and the boat was actually resting on the bottom upside down," he said.

Flight Lieutenant Chris Marriot battled to rescue the sisters

"They told me that there were two women trapped and that people had tried to dive under the boat to reach them but had been unable to do it."

As he climbed up on to the boat he heard shouting from inside. One of the women said she was all right but her sister was unconscious.

"She had actually dived under the water to pull her sister up and was standing up supporting her sister and keeping her head above water. And she was trying to resuscitate her.

"I tried to dive underneath but I couldn't get to them."

After an inshore rescue boat and a fire brigade crew arrived, they decided to cut a hole through the hull.

"It took us about 20 minutes to cut a hole and I was able to get my head and shoulders in and pull the ladies out and they were winched on to the helicopter and taken to hospital."