The temperature climbs a few degrees - the sun comes out for the first "heat wave" of the year - BBQ get's lit, shorts on, tops off, suncream on - everything looks rosey in the garden.
Time for a walk down by the river or a picnic by the lake - it's 21c for goodness sake
Even when the weather is warm, the waters around the UK remain extremely cold throughout the year. If you fall into the water or dive in for a swim, you are likely to suffer from cold water shock – your breathing increases uncontrollably meaning that you are more likely to take water into your lungs and drown.
Don't swim in rivers or lakes particularly on hot days - it could be the last thing you ever do!