An eyewitness said a teenage girl and a woman died after the tide came in while a group of people were on a sandbank near a holiday park.
Essex Police said officers were called by the ambulance service at 5.35pm on Wednesday after reports that several people had got into difficulty in the water near Seaview Avenue, West Mersea, Essex.
The force confirmed on Thursday morning that a teenage girl and a woman in her 40s had died.
A third person remains in hospital in a serious condition and two other people are in hospital with non-life-threatening injuries, police said.
The force said they believe those involved in the incident knew each other.
The East of England Ambulance Service said one patient was transported by air to hospital and a further three patients were transported by road to hospital after the incident in the water at Sea View Holiday Park.
On Thursday, police officers have been seen walking along the beach near the incident, speaking with members of the public.
A family friend of the deceased, Mishba Uddin, of Colchester, told the press that the families involved in the incident had come down for the day from east London and Barking.
He said the father is a doctor from Bangladesh heritage, but he “only knows the grandfather very well”.
“They are a very nice and likeable family,” added Mr Uddin.
Beach hut owner Beverly Smit, 60, of West Mersea in Essex, said she heard the emergency response and went to see what was going on.
“By that time they were re-sussing the person on the beach, the helicopters were still in the fields,” said Ms Smit, who works for a local dentist.
She said that she spoke to a day tripper who had rented a neighbouring beach hut and had tried to help.
“He saw and they were calling for help and the children and adults were out there,” she said. “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 yesterday was particularly quite cold.”
The beach hut owner added that she and members of her rowing club have helped save people in the sea previously.
Ms Smit described the incident as a “massive scale of rescue and tragedy really”.
A woman, who did not give her name, said the incident at West Mersea happened at high tide.
She told the Press Association: “The tide was a lot further in than it is now and that sandbank, they were on there and the tide came in.
“It comes in very quickly and you have to be careful.”
Speaking while sitting at a beach hut, the woman said there was “quite a commotion” and “just total panic”.
Discussing the aftermath of the incident, she said there was a “helicopter coming in, the helicopter circling and sirens going everywhere”.
The eyewitness added: “That was quite alarming, for a small place like this, I think that’s why so many people are shocked.
“I mean the beach was packed with people, they were coming down to see what was going on.”
The woman said the emergency services “worked diligently” and described the incident as “absolutely tragic”.
A worker at a nearby diner, who did not give her name, said some of their teenage staff had tried to help.
Carl Powling, a councillor for Colchester Council, said his daughter is one of three teenage waitresses at a local cafe who tried to help.
He told the Press Association: “Three teenage girls, the waitresses in there, actually came out.
“One of them was on the phone directing the police where to come, the other two were on the beach assisting with the first aid, keeping people away from filming it as well.
“Very level-headed, absolutely amazing for that age group.”
He said they helped until the coastguard arrived and there was a “phenomenal speed” of response.
Emergency services had urged people to avoid part of the beach in Essex after what the coastguard rescue team described as a “tragic incident”.
Detective Chief Inspector Al Pitcher said: “Today, our thoughts remain with the family members who are coming to terms with their loss.
“We are grateful to the people who have already spoken to us and helped with our inquiries.
“We would ask anyone who was at the beach at the time, and who has not yet spoken with officers, to please get in touch, to help us understand what happened.”
An East of England Ambulance Service spokesperson said: “Following reports of multiple people in distress in the water at Sea View Holiday Park, West Mersea yesterday evening, we dispatched multiple resources including our Hazardous Area Response Team, critical care paramedics and two air ambulances to the scene.
“Sadly, despite best efforts, two patients could not be resuscitated.
“One patient was transported by air to hospital, and a further three patients were transported by road to hospital.”
Anyone who was in the area at the time and has information or footage which may assist the police investigation is asked to get in touch on 101 and quote incident 1090.
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