Confident swimmers are more likely to venture into dangerous waters, according to the Royal Society for the Prevention of Accidents.
A spokesman said: "Learning to swim does not necessarily save your life.
Kids will swim in rivers because they can swim but a lot of swimmers overestimate their abilities. It is largely the mid-teens to the mid-30s who go into dangerous waters and drown."
Rospa believes children need to be alerted to the dangers of rivers and ponds where cold water, currents and reeds can carry them under. "The only safe place to swim is in a properly supervised swimming pool or a properly supervised beach," the spokesman said.
Last year, some 60 Britons drowned abroad, eight of them children, and mostly in Mediterranean waters, either around pools or in the sea.