The presenter's body was limp with her head lolling as she was pulled from the water at Lake Windermere after clinging on to her support team's kayak to complete her crossing.
Organisers of the seven-day challenge assured fans that McCall - who was crying and struggling to breathe during the swim - was in no danger as she made her way across the lake.
The presenter's pain and suffering was etched across her face as she had difficulty managing anything other than a doggy paddle and was unable to put her head out of the water.
McCall, 46, said afterwards that she even hoped for divine intervention to get her through.
She said: "I got so desperate I looked up to God. If there was anyone up there I needed their help."
But she added that she did not feel unsafe as she crossed the lake.
Viewers saw her brave the water live on BBC1, sobbing and red-faced before even taking the plunge.
When she eventually reached her target, her flopping body was lifted from the water and carried to a nearby hotel as the charity's team warmed her up.
Minutes later she was seen smiling and about to get her hands around a mug of hot chocolate in a photograph posted on Twitter, with her core temperature on the way back up.
And before long she was on a bike for a 65-mile leg in her attempt to travel 500 miles under her own steam as she heads from Edinburgh to London by running, swimming and cycling.
She said of her lake crossing: "It was such a struggle, but I have such a great team around me, I couldn't have done it without them."