Two boys are in hospital after being pulled from the sea at Blackpool on Monday as police issue an urgent heatwave warning.
The boys, aged 12 and 14, were pulled from the sea near Central Pier by two teenage girls at roughly 6pm on Monday.
They were given CPR by security staff before being taken by ambulance to hospital, a force spokesman said.
The eldest boy is in a critical but stable condition at Alder Hey Children's Hospital in Liverpool, while the 12-year-old is at Blackpool Victoria Hospital where he is described as poorly but not critical.
Superintendent Chris Hardy has issued a stark warning to anyone who is thinking of swimming in open water during the UK's sweltering heatwave.
"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," he said.
"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."
The warning came as the UK recorded temperatures over 40C for the first time ever, with 40.2C reported at Heathrow at 12.50pm today, according to provisional Met Office figures.
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.
HM Coastguard told the BBC they were responding to "a report of multiple people in the water".
A spokesperson said: "Five people have been rescued, with the search still ongoing for a sixth male."
Meanwhile, at least 11 people have died over the past week from drowning and "heat stroke" during the UK's longest heatwave for four years.
On Thursday, Emma Louise Powell, 24, died in Conwy Morfa, north Wales, after she and two other paddle boarders got into difficulties at the popular beauty spot, prompting a major search operation.
On Saturday, Kalen Waugh, 16, drowned after taking a dip with friends in Salford Quays, Manchester, with police also called to Ardsley Reservoir in Wakefield, near Leeds, after reports of a man getting into difficulty in the water.
The body of a 50-year-old man was recovered there following day after a 24-hour search, and a 37-year-old man was rushed to hospital after emergency services were called to a medical incident on Brighton beach, shortly after 7pm, but he sadly died.
On Sunday, 13-year-old Robert Hattersley died after getting into trouble at the River Tyne near Ovingham in Northumberland.
A 51-year-old man, named locally as Stuart Bailey, also died after his kayak capsized on the River Morey in Scotland. Another man in his 50s died after falling from his boat and getting into difficulties in the water in County Kerry, Northern Ireland.
And another man, whose identity is yet to be confirmed, died after reportedly collapsing from heat stroke amid extreme temperatures in East London.
And yesterday, a 16-year-old boy died after getting into difficulty in Bray Lake, near Maidenhead, Berkshire. A man was also confirmed dead shortly after being pulled from the water at the Cotwswold Water Park.
The Met Police were also alerted by London Ambulance Service to reports that a 14-year-old boy had entered the water at Tagg's Island in Hampton.
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.
