People cooling off during the heatwave have been urged to stick to supervised water after a spate of accidents left three teenage boys dead.
As the temperature soared past 30C (86F) in the first week of the school holiday, the Royal Society for the Prevention of Accidents (RoSPA) urged people to stay away from rivers, quarries and lakes and head to beaches, lidos and swimming pools instead.
Aaron Burgess, 14, died while cooling off in a stretch of the River Avon between Bristol and Bath on Tuesday, while a 15-year-old boy, named as Daniel James McCullagh, died in the River Nene near his home in Woodford, near Kettering in Northamptonshire.
A search for another 14-year-old who went missing after falling into the water at a quarry in the West Midlands was called off after a body was discovered.
David Walker, head of leisure safety at RoSPA, said: "We understand the temptation to want to go swimming in rivers, quarries and lakes, especially during the recent hot weather.
"Among the hazards to consider at inland waters are that, even on a hot day, water might be a lot colder than you were expecting and there may be strong currents and underwater debris that you cannot see from the bank.
"Consider how you are going to get out of the water before you get in, and be honest about your ability to look after yourself, and others, in and around water. RoSPA's advice is to go swimming at properly supervised sites such as beaches, lidos or swimming pools, although we appreciate that not everyone can get to these locations."
Large parts of the UK sizzled in the sunshine on Wednesday as the temperatures soared past 30C for the second day in a row. Charlwood, in Surrey, saw temperatures reach 30.3C (86F), beating Tuesday's high of 30.2C recorded at Eton Dorney Lake in Buckinghamshire to make Wednesday the hottest day of the year so far.
The Olympic Park in east London also saw temperatures reach 30.1C and Thursday could see similar readings.
Billy Payne, forecaster with MeteoGroup, the weather division of the Press Association, said: "It is going to be another hot day with plenty of sunshine in the South and temperatures should get up to about 31C (87.8F) or so in the London area. Further north will be more unsettled with patchy rain but it will still be warm."