A 15-year-old girl died after she spent up to 30 minutes in chilly seas despite the efforts of a dog walker who tried to revive her, a lifeboat volunteer said.
The girl was with a group of teenagers on the north pier at Cullercoats Bay, North Tyneside, on Saturday evening when a large wave hit and she was washed into the sea.
A 999 call sparked a major search operation involving the RNLI, whose station is yards from the pier, Northumbria Police officers and their helicopter, ambulance crews and the Tynemouth Volunteer Life Brigade (TVLB).
The RNLI crew searched rocks around the north pier for about 15 minutes then spotted a man in a high-visibility jacket with two dogs, about 100 yards away on the beach, and he was administering first aid.
RNLI man Curtis Dunn, who worked on the search, said fellow crew members jumped off their launch to help the man.
He said: "We don't know this member of the public, he was just down walking his dogs. We would like to pass on our thanks to him.
"We presume he had just come back from work, he was still in his hi-vis and that helped us as we were able to spot him quite quickly
"He was in the right place at the right time. Unfortunately, it was not the outcome that we all hoped for."
Peter Lilley, a captain with the TVLB whose team worked on the shore to spot the teenager, said: "She was unconscious and not breathing when she was found in the water line and CPR was started straight away."
The spot was popular with teenagers and Mr Lilley urged people to take care, saying: "We think she was in the water between 25 and 35 minutes.
"We are not quite sure what happened, it is possible she was knocked unconscious as there's lots of rocks.
"Obviously the water temperature is very cold and that will have made her prospects quite slim as well."
It was bitterly cold on the coast and the sea was choppy, a local resident who saw the rescue operation said.
Dr Clare Guilding, a lecturer at Newcastle University who lives in Cullercoats, was alerted by the police helicopter hovering overhead.
She said the beach was known locally as a popular place for teenagers to congregate.
Dr Guilding added: "The weather was clear but it was absolutely freezing. It said it was minus 2C (28.4F) but once you get out on the front it was just bitterly, bitterly cold.
"The sea was choppy but I have seen it worse than that before."
Before the girl died, Chief Inspector Paul Knox said: "People should always be careful around water and if the sea is looking rough with waves pounding onto the pier, we would advise people to be particularly cautious and keep their distance."
Anyone with information about the incident should call 101.
The RNLI released dramatic black-and-white footage of the search, with crew members shining torches onto the rough seas, in the hope of spotting the teenager.