The Royal Life Saving Society (RLSS) UK said warmer weather could give a false sense of safety, with many people drawn to the water.
Sea temperatures across the UK are still low for the time of year, ranging from about 9C in Scotland to 13C in the South West, increasing the risk of cold water shock.
Ashley Jones of the RLSS, said: “As air temperatures rise, it’s important people don’t underestimate the risks associated with entering open water.
“Even during a heatwave, water will remain dangerously cold and cold water shock is a very real danger.
“Sudden immersion can cause an involuntary gasp, rapid breathing and panic, which can quickly lead to a loss of control in the water, even for strong swimmers.”
He urged people to swim at lifeguarded beaches, supervise children closely and never enter the water to try to rescue someone in trouble, instead calling 999 and throwing something that floats to help keep them safe.
