Doing as little as possible in the first minute or 90 seconds of immersion helps people regain control of breathing and increases their chances of survival, the charity says.
The new advice states that floating for 60 to 90 seconds would allow cold shock to pass, and advised the public not to attempt to swim as it increased the chance of drowning.
Cold water shock is caused when a person suddenly finds themselves in water less than 15C, and is considered to be one of the most common causes of death after falling in the sea.
Professor Mike Tipton, an expert in Human and Applied Physiology at the University of Portsmouth, said: ‘We often rely on our instincts but our instinctive response to sudden immersion in cold water - gasping, thrashing and swimming hard - is potentially a killer.
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‘It increases chances of water entering your lungs, increases the strain on your heart, cools the skin further and lets air escape from any clothing, which then reduces buoyancy.
‘Although it’s counter-intuitive, the best immediate course of action in that situation is to fight your instinct and try to float, just for a short time.
‘The effects of cold water shock will pass quite quickly, within 60-90 seconds. Floating for this short time will let you regain control of your breathing and your survival chances will greatly increase.”