Yesterday's incident unfolded at about 8:30am when the man, aged about 60, suffered a heart attack and stopped breathing.
He was resuscitated by doctors before being flown to hospital in Newcastle Upon Tyne, where his condition was critical last night.
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Hide AdAbout 10,000 people were taking part in the three-day Great North Swim which involves half-mile, one-mile and two-mile swims on Windermere in groups of up to 300 people.
Many of the swimmers are amateurs raising money for charity.