The presenter's pain and suffering was etched across her face and she had to cling on to her support team's kayak as she was overcome with the effort and the cold of her open water swim.
McCall, 46, was limp with her head lolling as she was dragged from the water after reaching a jetty as concerned organisers rallied around her, carrying her clear and wrapping her up.
But minutes later was seen warming up in a hotel room and about to get her hands around a mug of hot chocolate in a photograph posted on Twitter, and her core temperature was said to be on the way back up.
The presenter is attempting to travel 500 miles under her own steam as she heads from Edinburgh to London by running, swimming and cycling.
Within hours of setting off on Saturday morning her team was concerned she was exhibiting signs of hypothermia as she battled severe weather during a tough 130-mile stint in the saddle.
But after being examined she was cleared to continue with her seven-day journey, the BT Sport Relief Challenge: Davina - Beyond Breaking Point.
Yesterday she scaled Scafell Pike, where she again faced treacherous weather, trudging through snow and negotiating her way through the cold mist.
In previous years brave celebrities have put themselves through other draining pursuits for Sport Relief. David Walliams picked up stomach bugs during his swim down the River Thames, as well as developing sores from his wetsuit, and two years ago John Bishop was left in agony amid worries about stress fractures as he did back-to-back marathon distance runs en route from Paris to London.
McCall later indicated she was able to continue with her journey and was poised to have another stint of cycling.
In a message posted on Twitter, she said: "I'm OK. That was hairy. Thank you for all your support. I'm about to get on a bike."
McCall's passage across the lake was one and a half miles in length with the water as deep as 183ft (56 metres), and she was accompanied by her trainer Professor Greg Whyte for the swim in temperatures of 5C (41F).
She said she struggled to do anything other than the simplest of strokes.
"The moment I got in the water, my chest felt crushed - I couldn't do front crawl, I couldn't put my head under water, I couldn't breathe," McCall said.
"One thing Greg told me that was pulling me through was to think of the building ahead of us as a magnet, drawing us towards it.
"When I was near the end and really struggling, I got so desperate I looked up to God - if there was anyone up there I needed their help! I kept trying to lift my arm to swim, trying to pull the crawl out of the bag, but I couldn't even get it up. I ended up doing doggy paddle!
"I imagined my sister holding me and pulling me through it. It was such a struggle, but I have such a great team around me, I couldn't have done it without them. Although I'm shattered I never felt unsafe, I'm in the best possible hands."
Prof Whyte said: "I was in the water throughout the whole swim to ensure Davina's safety. It was very tough and very cold, but I can reassure people that she was safe throughout and never in any real danger. I'm full of admiration for her determination to finish this swim and get back on her bike."
She has to complete a 65-mile cycle before her third day of the challenge is over.