A 14-year-old boy is missing and is believed to have drowned after getting into difficulty in the Thames in west London.
Emergency services and the Government have reiterated urgent warnings about the dangers of trying to keep cool after several tragedies in waterways and reservoirs during the heatwave.
At just before 5pm on Monday, the Metropolitan Police were alerted by London Ambulance Service to reports that a child had entered the water at Tagg's Island in Hampton, Richmond.
Despite a search by officers, paramedics, London Fire Brigade and members of the public, efforts to find the boy were unsuccessful and the rescue operation was brought to an end at 6.40pm.
Efforts to recover his body will continue into the evening, with the boy's next of kin aware and being supported by specialist officers.
Superintendent Richard Smith, from the South West Command Unit, said: "Despite the very best efforts of all involved, we must now sadly conclude that this young boy has died.
"His death is a tragedy and I cannot begin to imagine what his family will be going through. All our thoughts are with them.
"I know that on days like today when temperatures are at a record high, it might look appealing to jump in and cool off in rivers, reservoirs, lakes or other open water.
"Please don't. The dangers are real and this evening in Richmond we have seen the terrible consequences of what happens when it goes wrong.
"To young people in particular, I would urge you to be the person in your group of friends who says no and reminds others about the dangers.
"Your intervention could save a life and save another family from experiencing such an awful loss."
'Open water is a lot colder than you think'
Fire Brigade assistant commissioner Charlie Pugsley said: "Our thoughts are with the teenager's family and friends. Please don't go into open water during this heatwave, it can be incredibly dangerous as the water is a lot colder than you'd expect.
"Anything below 15C is defined as cold water, and on average the UK and Ireland sea temperatures are just 12C. Rivers such as the Thames are colder - even in the summer."
Meanwhile in Essex, a swimmer is missing at sea and five others have been pulled from the water after a "serious" incident close to Clacton Pier.
Emergency services have been called to the scene and a major search and rescue is underway to find the missing male swimmer.
We're on the scene assisting emergency services colleagues with a serious and ongoing incident close to #Clacton Pier.
— Essex Police (@EssexPoliceUK) July 19, 2022
There is a significant emergency services presence in the area while the incident is being dealt with.
We'll provide an update as soon as we practically can. pic.twitter.com/f6GjiYmwdA
Nigel Brown, communications manager for Clacton Pier, said: "There could have been a group of up to eight in the water originally. Can I make it clear nobody jumped off the pier."
"These people were on a beach to the east of the pier, went in swimming and the tide and the currents dragged them towards, under the pier. Pier staff threw lifebelt rings over the side of the pier to help some of the casualties and they had about four of them on those."
"Eight of them were in the water but my understanding is four or five got into difficulty."
'Enjoy the beach, respect the water'
Two boys are in hospital after being pulled from the sea at Blackpool on Monday, Lancashire Police said.
The boys, aged 12 and 14, were pulled from the sea near Central Pier by two teenage girls at about 6pm and give CPR by security staff before being taken by ambulance to hospital, a force spokesman said.
Police said the 14-year-old boy is in a critical but stable condition at Alder Hey Children's Hospital in Liverpool and the 12-year-old is at Blackpool Victoria Hospital where he is described as poorly, but not critical.
Superintendent Chris Hardy said: "This incident once again shows all too clearly the dangers that open water can present and I have no doubt that if it hadn't been for the brave and swift actions of the two girls who helped pull these boys from the water, and the security staff who gave CPR, that we would have been dealing with at least one fatality.
"Over the last couple of weeks we have very sadly seen a number of youngsters across the country who have died after getting into difficulty in open water and a number of families left bereaved.
"As this very warm weather continues, at least in the short term, I would again plead with people to enjoy the beach but to respect the water and take care around swimming in the sea, especially if you aren't prepared or are not a strong swimmer."
It came after the family of 13-year-old Robert Hattersley said they were "absolutely devastated" after he died when he got into trouble in the River Tyne in Northumberland on Sunday.
Emergency services also confirmed the deaths of a 16-year-old boy in Salford Quays, Greater Manchester, a 16-year-old boy in Bray Lake near Maidenhead, Berkshire, and a 50-year-old man in a reservoir near Leeds in similar circumstances.
Detective Inspector Phil Hughes, of Leeds CID, said: "This incident (in Leeds) serves as a timely reminder about the dangers of swimming in open water."
Northumbria Police said Robert's death in Ovingham "does act as a poignant and timely reminder as to the potential dangers presented by water".
The Crawcrook teenager's family said: "It is impossible to put into words the heartbreak we are feeling. Robert was so kind and loving. We are absolutely devastated by what has happened.
"He brought a smile to so many people's faces and he will be missed by absolutely everyone who knew and loved him.
"We'd like to thank all the emergency services who worked so hard to try to find Robert, as well as everyone who has been in touch passing on their messages of condolence."
