At least 17 people have died in the water since Saturday during the heatwave, according to a charity.
Warnings of the danger of open water have been issued as temperatures climbed to 32C in the July heatwave.
The Royal Life Saving Society UK (RLSS UK) said it is aware of 17 incidents of accidental loss of life in the water between July 17 and July 20.
There have been two further reports of people going missing in the water who are yet to be found.
Lee Heard, RLSS UK’s charity director, said: “Whilst we recognise how tempting it is to cool off in the UK’s beautiful waterways, they hide hazards that tragically take lives each year and we urge the public to use caution when entering the water, getting acclimatised to the water temperature before jumping in.
“The difference between the air temperature and water temperature can literally take your breath away; this is called cold water shock. It is silent, invisible and deadly.
“Water can also hide debris, strong currents and sudden changes in depth that can catch out even the strongest swimmers.
“The Royal Life Saving Society UK urge everyone to learn vital skills and knowledge to prevent future accidental drownings.”
All the incidents have taken place in England, except for one which happened in Northern Ireland.
West Yorkshire Police also issued a warning about swimming in open water in hot weather.
The hottest day of the year soared to temperatures of 32.2C at Heathrow Airport in west London.
Heavy rain and thunderstorms are set to bring an end to the heatwave over the weekend.
Extreme heat warnings have been put in place until Friday.
Stephen Dixon, a spokesperson for the Met Office, said: “Late on Friday, a band of heavy and thundery rain will migrate from the south-west of the country.
“There are yellow weather warnings for rain on Saturday and Sunday for southern areas of the UK.”