A father, mother and their teenage daughter who died off the Sussex coast on Tuesday have been identified by police.

Sussex police named the victims as Hassan El-Khawas, 55, his partner, Zeina Alayn, 37, and their 14-year-old daughter, Sara El-Khawas. A younger girl who was also part of the family remains in hospital in a critical condition. She has not been named.

The Palestinian embassy in London said El-Khawas was a British man of Palestinian descent.

Police, paramedics, coastguard crews and a lifeboat attended the scene at Shoreham-by-Sea, where four people were pulled from the water.

Sussex police said the cause of the tragedy was still being investigated. It could not confirm reports that the older girl got caught in a fishing net. In a statement, the force said: “Following review of CCTV footage and initial witness accounts, the current hypothesis is that this was a tragic accident, with no third-party involvement.”

Adnan Hmidan, the chair of the Palestinian Forum in Britain, said: “According to the information available to us, the family encountered strong currents while at sea. Their daughter, Sara, became caught in a fishing net, and her father attempted to help free her. As the waves and currents intensified, her mother, Zeina, also entered the water to try to save them, followed by their younger daughter.”

Hmidan said two other children from the family are now in the care of their paternal uncle.

He added: “The Khawas family are British citizens of Palestinian origin, with roots in Rashidieh refugee camp in Lebanon. Like many Palestinian families, members of their extended family have lived in Britain for many years and have built a strong and close-knit community here.

“Our thoughts and prayers are with the entire family, particularly the children, who are facing an unimaginable loss. We ask everyone to respect their privacy and dignity at this painful time.”

Andy Burnham said the incident was “a terrible family tragedy” and a reminder of the dangers of open water swimming.

There was a huge emergency service response to the incident on Tuesday afternoon, when police were told four people had got into difficulty in the water.

Sussex police said they believed all four entered the water to swim. Ch Supt James Collis confirmed that a man, woman and a teenage girl had died at the scene.

Asked if the government needed to do more on water safety, Burnham said: “Always. It’s so sad to hear what happened yesterday, and obviously we think there’s a young girl still in the hospital fighting for her life, and we send our best wishes and love to wider friends and family. But a real tragedy, a real terrible family tragedy.”

Speaking on a visit to north-east England, Burnham added: “Obviously, we will look in detail at what happened. The thing I can say today, and there is still, I think, a further heatwave to come in this summer period, it is just to remind people about the dangers of open-water swimming. The coldness, even when it feels hot outside, it can shock people, and that can cause the difficulties. So people just need to remember that at all times.”

Two air ambulance helicopters and a critical care team were sent to the scene and Shoreham RNLI said its lifeboat had been called out at 3pm.

Burnham paid tribute to those who tried to save the family. “I do need to praise the emergency services, who I know were on scene very quickly yesterday. They’ve had a busy summer, but they were there, and we thank them. As I say, just huge sympathy to the friends and the family,” he said.

There have been a series of water-related deaths across the country in a summer of record-breaking heat. On Saturday, a 12-year-old boy drowned in the River Lea, near Hackney Marshes in north-east London.

In April, a woman and her young child died after getting into difficulty in the water in west London. Both died at the scene in Elthorne Park, Ealing, the Metropolitan police said. There have been at least 35 drowning deaths, including eight in July, according to a study by the London School of Hygiene & Tropical Medicine.

Joss Loader, the chair of Shoreham Beach Residents’ Association, said: “There’s just a really strong sense of shock, obviously people are really concerned for the young girl. A small, tightly knit community with a very deep sense of shock and that sense of disbelief you get after a tragedy because it has come out of nowhere.”

Shoreham beach is a spit of land with water on three sides from the River Adur, the estuary and the sea, and Loader said she understood the “tragic events” had happened in the estuary.

“It’s school holidays and we have had a wonderful summer so the beaches have been packed with families,” she said. “People swim from those beaches even in the winter, generally it is considered as safe as the sea can be. You wouldn’t be swimming there and think you were taking a risk.”

Anna Peacock, 81, who lives near Kingston beach across the water from Shoreham Fort, said emergency services were on the scene very quickly.

“People come down to swim, particularly in the summertime and I don’t think they realise there’s a riptide along here,” she said. “It’s very inviting to come to the sea, but the tide could carry you out in seconds, and I don’t think people have realised that.”