I stop at Portishead straight after Clevedon Marine Lake, and the water is gloriously warm in comparison. Jacuzzi warm to my salt-blushed skin. The first thing that strikes me is the colour of it all; bright orange, yellow and blue decorate the walls and sunbathing steps around the pool. Two diving boards jut out at one end. It is bright and friendly, the kind of place to have its own dedicated champions. It feels like a very special place.

I swim lengths, feeling almost confused by the order. After the roaring wilderness I’ve become used to, the straight lines seem strange and tight. But it is nice to really swim, to stretch out and feel the muscles lengthen. It’s nice to not feel sticky with salt. And it’s nice to see the community that a pool like this fosters; two pregnant women swimming slow lengths, two children splashing and squealing, a classic hole-in-the-wall cafe selling ice-lollies and chips.

After I’ve swum and stretched and lingered in the bath-hot warmth, I climb the sunbathing steps for a view out over the sea. The rectangle of water sits so close to the sweep of the bay and it is this juxtaposition of natural and manmade, free and ordered, unpredictable and safe, that makes it as special as it is. The sea is wonderful and awe inspiring, but here you can swim in safety every day.

I lie on my back and idly kick my legs, floating easily, my head resting on the concrete side. Light pours out from behind a cloud, setting white rainbows dancing everywhere, then turns away again behind the next cloud, engaged in a constant game of hide and seek that transforms the pool and all its colours. This is the magic of outdoor swimming;  you watch the sky and, even while you are not watching it, you know what it is doing because of the changes in the water around you. It reminds you of the harmony of the natural world and the fundamental joy of witnessing these everyday moment-to-moment changes.

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