“So lie on your back with you head back, with your ears submerged, use your arms and legs to stabilise yourself and just let your breathing come back to a normal breathing pattern.
“And when you’re feeling confident that you’ve gotten used to the water and your breathing’s normal, you can shout for help or swim to save yourself.”
On Tuesday, the UK recorded the hottest day in May with temperatures reaching 35.1C at Kew Gardens in London.
For two days in a row, parts of south-east England went above 34C.
Nearly all weather stations across England and Wales also broke their local May temperature records on Monday and Tuesday.
The Met Office says a heatwave happens when an area reaches a certain high temperature for at least three days in a row.
Wednesday brings a reprieve from the record-breaking temperatures and has likely peaked, as cooler air moves over northern and eastern areas of England.
The Met Office has also issued a yellow thunderstorm warning for much of Wales, south-west England and parts of the Midlands from 15:00 to 23:59 on Wednesday.
It could bring torrential downpours of rain, frequent lightning, large hail and strong, gusty winds.
But over the next few days it will still remain hot in many areas and health officials confirmed heat health alerts would be extended for 24 hours.
The UK Health Security Agency (UKHSA) said amber alerts for the South West, South East, London, East and West Midlands, and East of England will remain until 17:00 on Thursday, with yellow heat-health alerts for the North West and North East.