At least 17 people have died in the water since Saturday amid scorching temperatures across the UK, according to a charity.
Swimmers looking to cool off are being urged to take care in open water as 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.
All the incidents have occurred 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 after they recovered the body of a 15-year-old boy, from Stocking Lane in Knottingley, who was pronounced dead at the scene on Tuesday.
Merseyside Police said that a man had died and two others were taken to hospital following an incident at Crosby Beach on Tuesday.
The force said officers were called at around 7.10pm following reports of three men in their 20s getting into difficulties in the sea.
England reached its hottest temperature of the year on Tuesday when 32.2C was recorded at Heathrow Airport in west London.
Thursday saw a top temperature of 31.1C in England, in Derbyshire, while the mercury hit 31.4C in Armagh, Northern Ireland, 31.2C in Ceredigion, Wales, and 29.3C in Dumfries and Galloway, Scotland.
The 31.4C reading in Armagh, which was reached at 3.20pm, is Northern Ireland’s highest temperature on record, while Thursday’s highs marked the hottest day of the year so far in Scotland and Wales.
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.”
Heavy rain and thunderstorms are set to bring an end to the heatwave over the weekend.
The Met Office has issued a yellow warning for rain for much of central and southern England from midnight on Saturday until the end of Sunday, with the potential for flooding and transport disruption.
Extreme heat warnings which have been in place for south-west England and parts of Wales will expire on Thursday night, while the heat warning for Northern Ireland is in place until the end of 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.”
According to the Met Office, there is the potential of up to 100mm of rain in some locations over the weekend, such as either side of the Bristol Channel.
However, Northern Ireland and Scotland look set to hold on to higher temperatures for longer, with both expected to be relatively dry and fine with temperatures in the low 20s.