Ambulance bosses are warning people not to jump in rivers and lakes to cool off during the heatwave after a string of fatalities.
They say unseen debris and underwater undercurrents can lead to treacherous conditions in the water, and cold water can cause hypothermia even on a hot day.
Yorkshire Ambulance Service (YAS) said several people had already died this year around the UK after jumping in rivers, reservoirs, ponds, quarries, lakes and canals.
And traditionally, the incidents increase after schools break up for the summer holidays.
Paul Mudd, YAS acting director of operations, said: “It may be very appealing to jump into the water to cool off on a warm summer’s day but people need to be aware of how dangerous it really is.
“Although it is banned, people are also often tempted to swim in reservoirs without realising that there is automatic equipment located under the surface which can operate without warning and cause dangerous hidden currents.
“Furthermore, the temperature of deep water is much colder than people would expect and, even on a hot summer’s day, rarely gets above freezing. This is cold enough to take your breath away, which is the body’s natural reaction and cannot be controlled, possibly leading to panic and drowning.
“Several lives have already been lost this year across the UK and we don’t want to see anyone else hurt or injured so the message is simple - enjoy the summer, stay out of open water and stay safe.”