Witnesses say a heroic day-tripper managed to haul two children back to shore during an incident where a mother and daughter drowned.
Beverly Smit, 60, described the unnamed rescuer swimming out to sea three times as the tragedy unfolded in West Mersea, near Colchester, on Wednesday evening.
Shally Rahman, 41, and Samiha Rahman, 15, were trying to rescue a seven-year-old boy from the water when they got into difficulty and died.
Essex Police said the group, from Barking, east London, were all related to each other and the boy remained in hospital, fighting for his life.
Recalling the moment the “Good Samaritan” sprang into action, Smit said: “They were calling for help and the children and adults were out there.
“He swam out, I think, three times. Two of the children he brought back were not in good condition.
“It’s not an easy swim when the tide’s coming in – you’re swimming against the tide. The water [on Wednesday] was particularly quite cold.”
The mother and daughter died at the scene at about 17:35 BST, while the young boy was airlifted to the Royal London Hospital.
Three others were taken by ambulance to another hospital.
People have described how bystanders rushed to the water to help, including teenagers who worked at the nearby Seaview Beach Diner and an off-duty paramedic.
Leonie Powling, 19, had been working in the nearby cafe and recalled watching people administer CPR to those pulled from the water.
“We wished we helped a bit more, but we did what we could,” she said.
“Now knowing that people have died, it’s a lot sadder that we were there and kind of witnessed it.”
Residents living in nearby flats rushed to the scene with blankets, hoping to help both emergency crews and those who had been in the water.
Shally Rahman’s brother-in-law, Matiur Rahman, said his family were “speechless” about the incident.
“We are very shocked. We can’t explain [what has happened],” he said, speaking to the BBC from Bangladesh.
He said his niece was a “very, very good person” and his sister-in-law was “a very helpful person”.
The BBC understands the boy may have been on a sandbank when he was cut off from the shore, sparking the rescue efforts.
Detectives have been trying to piece together how the tragedy unfolded and have appealed for witnesses to come forward.
Det Ch Insp Al Pitcher said: “We are working through speaking to dozens of witnesses and taking their accounts, and we are building a picture of what happened.
“These inquiries are ongoing and I need anyone who has any information to get in contact.”
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Mum and daughter who died at beach were trying to save boy, 7
Mother and daughter who died at Essex beach were trying to help boy, police believe