A woman and her 15-year-old daughter who died after getting into difficulty in the water on the Essex coast were trying to help a seven-year-old boy, police have said.
The child remains in a critical condition in hospital after the incident at West Mersea near Colchester on Wednesday evening. Three others were taken to hospital.
The victims have been named by a family member as Shally Rahman, 41, and teenager Samiha Rahman. Police said all the people involved knew each other.
Shally's brother-in-law Matiur Rahman said the family was enjoying a seaside holiday when a "big wave came and took them".
"My whole family, we are, like, speechless," said Matiur Rahman, who used to live in Bradford, but spoke to the BBC from Bangladesh.
"We are very shocked. We can't explain [what has happened]."
He said his niece was a "very, very good person" and his sister-in-law was "a very helpful person".
Local people have described bystanders rushing to the water to help, including teenagers who worked at the nearby Seaview Beach Diner, and at least one off-duty paramedic who was walking their dog.
Prime Minister Andy Burnham, speaking on a visit to Harlow in Essex, described the deaths as "very sad" and thanked emergency services for their efforts.
Essex Police - which was called by the ambulance service at 17:35 BST to the water near Seaview Avenue - said the victims were from London.
"At this stage we believe they were trying to help a seven-year-old boy who had got into difficulties," said Det Ch Insp Al Pitcher.
He said officers had been speaking to "dozens" of witnesses to establish the circumstances leading up to the tragedy, and asked anyone else with information to contact the force.
The detective thanked first responders who worked "tirelessly" to try saving their lives.
The East of England Ambulance Service said the deceased could not be resuscitated "despite best efforts". A spokesperson said four more people in total were taken to hospital, including one by air ambulance.
Rita Coombs, one of the nearby residents who laid flowers at the beach, described the events as "absolutely horrific".
One man "couldn't speak" about what he had witnessed after returning from the scene, she added.
"I can't praise the emergency services [enough], it was beyond all belief, they worked so hard," said Coombs, 70.
"It's tragic - absolutely tragic."
Carl Powling, a Conservative Colchester city councillor for West Mersea, said the HM Coastguard and RNLI teams were "very quick off the mark" and "very professional".
His 19-year-old daughter Leonie Powling, who works at the cafe, was one of the teenagers who assisted at the scene.
She tried looking for a paddleboard she could use to get to the family in the water, and described watching people trying to rescue them and then administer CPR.
"We did what we could," said Leonie Powling.
High tide was expected at 18:47 that evening and Carl Powling said it could frequently "drag you out to sea".
"If you are not familiar with the area, it can be very dangerous," he said.
Reverend Jo Parrott opened St Peter and St Paul's church on Wednesday evening and Thursday to offer a place of prayer and support for anyone affected.
She said her husband responded to the incident as an RNLI volunteer after receiving a beep on his pager.
"Our hearts are just breaking for those we now know were lost and for their family and friends," she said.
Harriet Whiteside laid flowers at the beach with her son, which she had brought from her garden.
"Everyone is really upset," said the 36-year-old.
Claire Douglas, whose house backs on to the beach, told the BBC that "nobody" left the scene on Wednesday until "they'd done as much as they could".
"It's just so devastating," said a tearful Douglas.
"You just get caught up in the sort of panic thinking - 'there's nothing I can physically do' - you're just standing watching."
Additional reporting by Lewis Adams, Zoe Applegate and Tom Larsen-Wright.
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