Tinside lido is one of the most beautiful pools I’ve ever seen. Nestled up against the cliff, a huge crescent of turquoise and light navy stripes just put into open ocean. Off-shore islands break the boundless expanse of wind-chopped sea and a headland protrudes on one side to frame the scene.

The water is saltwater, un-heated and pulled into racing peaks and troughs by the wind. There is a fountain in the centre that shoots a plume of white water into the sky, a plume that then thunders down to hit the plinth, white water forming the rivulets of a waterfall. Swimming close by with your head under is like swimming through cloud, the wavy sun shafts on the striped lining disappearing along with the clear blue clarity of the water. Other swimmers bodies, the above water calls of children splashing – all fades into the white flurry of disturbed water.

This swim is also poignant for personal reasons. I started to do these swims to raise money for the Alzheimer’s society, a cause that I care about desperately. Today I thought that, for the first time in this collection of swims, my mum – who has Alzheimer’s – might join me. But when we get there she doesn’t know where she is and refuses to get changed. It’s a reminder of why I’m doing this and of the ugly reality of this disease – it isn’t filled with picturesque moments, but with compromises, with ever shifting expectations, with sadness but also hopefully with love.

I’m raising money for the Alzheimer’s Society. Please sponsor me at http://www.justgiving.com/swimbonnieswim