The RNLI has issued an urgent safety warning to those planning New Year's Day sea swims following the disappearance of two men off the Devon coast on Christmas Day.
Cold-water shock combined with harsh weather conditions creates serious risks for anyone entering water at 15C or below, the lifeboat charity has said.
Seas off the coast of the UK and Ireland currently average between 6C and 10C.
Gareth Morrison, the RNLI's head of water safety, said on Tuesday: "We know this time of year many people take part in festive and new year dips or continue to take part in regular winter swims."
Mr Morrison urged swimmers to assess their abilities and check sea conditions carefully before entering the water.
Those new to open water swimming should speak to their GP first, he added.
"With the sea temperatures still dropping and reaching their coldest around March, the effects of cold water, combined with weather conditions and any personal health issues, should be taken seriously before venturing in," Mr Morrison said.
"Avoid swimming alone; consider going with others or joining a group so you can look out for each other.
The RNLI has issued an urgent safety warning to those planning New Year's Day sea swims
|GETTY
"We'd recommend wearing a wetsuit to keep you warm and increase your buoyancy, together with a bright swim cap and tow float to make yourself visible to others and use in an emergency."
He also stressed that bystanders who see a swimmer in trouble should never enter the water themselves.
Instead, they should immediately contact emergency services.
"The most important thing to remember is if you are in any doubt stay out of the water and if you or anyone else does get into trouble in or on the water, please call 999 or 112 and ask for the Coastguard," he said.
SAFETY AT SEA - READ MORE:
The lifeboat charity's Gareth Morrison urged prospective swimmers to take the dangers of January 1's waters seriously
| PAThe RNLI alert comes after Matthew Upham, 63, and a second man in his 40s went missing during a festive swim at Budleigh Salterton in Devon.
Several other swimmers were brought safely to shore, with one taken to hospital as a precaution.
The two men are said to have entered the water after spotting a woman struggling in the sea.
Mr Upham and the second swimmer, who has not been publicly named, vanished while attempting to help her.
Matthew Upham (pictured) and another man remain unaccounted for after getting into trouble in stormy waters on Christmas morning
| YOUTUBERescue teams brought multiple people safely back to shore during the incident.
Paramedics assessed two of those rescued at the scene.
The coastguard search for the missing men was later called off.