It was a phone call Maxine Johnson will never forget.Playing at a park with her nine-year-old daughter in a heatwave 20 years ago, the charity worker got a call to rush to Pontsticill Reservoir, where her son, Reuben Morgan, and his friends had gotten into trouble while swimming in the sunshine.On arrival, she found a major emergency operation was underway with specialist search teams in the water at the beauty spot in the Brecon Beacons, while a helicopter hovered in the sky overhead.“I got told my son was missing,” she said. “My heart sank, and I just hoped and prayed he would be found safe. For three days, they searched; it was torture, and then they found his body. Words can’t describe. We were heartbroken, and still to this day the pain doesn’t go away.”Reuben, aged 15, was a fit teenager who was a good swimmer, played rugby and regularly went to the gym.Follow our latest coverage on the weather and warnings here“That was part of the shock of it all,” she said. “Reuben was super fit and someone you’d never think would get into trouble swimming.”Such is the danger of cold water shock. Reuben was camping with friends and celebrating after finishing his GCSEs (Maxine Johnson)It later emerged that Reuben, who was wearing only shorts and had been camping nearby after finishing his GCSEs, had tried to swim across the reservoir with his friends, but three-quarters of the way across, he disappeared. Now, two decades on, his mother is reminded of the tragedy during hot weather, and when news breaks on other teenage deaths in the water.Over the bank holiday weekend, when the highest spring temperature ever was recorded in the UK, four teenagers – three boys and one girl – died in separate water-related incidents. On Tuesday, the National Fire Chiefs Council issued a water safety warning for children over swimming in rivers, lakes and reservoirs.Pontsticill Reservoir, where Reuben died while trying to swim across the water in 2006 (Getty)“I feel for their [children’s] families,” Ms Johnson said. “I don’t know the exact horror they will be going through, but I know the pain, the heartbreak. That feeling of shock never leaves you. You never expect your grown-up child to drown in water. “But now having faced it, you know the dangers of swimming in remote spots. The hidden risk of going into cold water when it feels warm outside. The debilitating impact it can have on the body. If my son had it, anyone could.”And Ms Johnson, who has led campaigns for water safety ever since her son’s death, says there is one thing parents can do to lower the risk for their children.“Obviously, firstly talk to your children, share the dangers and find out where they are going,” she said. “But I think children should also be shown the floating position for if they get into difficulty in the water. If Reuben had known it, I think he’d have been here today.”Maxine Johnson is calling for more young people to learn a lifesaving float technique (Supplied)Last year, the RNLI joined forces with Ms Johnson to spread awareness of the lifesaving Float to Live technique, which involves a person tilting their head back, moving their hands and legs and attempting to relax, helping help them catch a breath until the cold water shock passes.“If children are taught swimming in school, it save many lives by sharing this technique,” Ms Johnson, who is chief executive of Age Connects Cardiff and the Vale, says.For now, Reuben’s family are looking ahead to marking 20 years since he died, on 28 June 2006, when they will be holding a fundraising social event in Merthyr Tydfil.“I think about him every day,” Ms Johnson says. “I just don’t want his life to be in vain, I know he’d have wanted us to help others.”For more information on the RNLI’s Float to Live campaign, click here