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.
Last year, 162 people died on the coastline around the UK, and the charity estimates that nearly half did not intend to enter the water.
A survey carried out showed that 40 per cent of respondents said their instinct would be to swim.
Only three per cent responded saying they would try to float - and the RNLI were trying to encourage greater awareness as the summer approached.
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.
"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."
He added: "Floating is not an easy skill in cold open water but most people can float, and the air trapped in their clothes as they fall in should make it easier.
"As little exercise as necessary can be undertaken to help stay afloat. The recommended floating position is to lean back in the water and keep your airway clear.
"Keeping calm will help maintain buoyancy. Some people find it helpful to gently scull with their hands and kick their feet to keep afloat. The main principle is to do as little as possible until you have control of your breathing.
"At this point you have a much better chance of avoiding drowning and surviving until you can swim to safety, call for help, or continuing to float until help arrives."
Young men have accounted for three-quarters of coastal deaths in the UK in the past five years, and the charity was hoping to reach that demographic in particular.
Last year, swimming, jumping in and general leisure use of the water accounted for a quarter of fatalities in rivers, coasts and canals.
RNLI Coastal Safety Manager Ross Macleod said: "The RNLI's volunteer lifeboat crews and lifeguards saved nearly 500 people from near-fatal incidents in 2016 and rescued thousands more but, sadly, they aren't able to reach everyone.
"If people in danger in the water can help themselves initially by floating and regaining control of their breathing, they stand a much greater chance of surviving."