I knew the tide would be low but I went to the beach anyway. I knew if it was completely unswimmable then I could re-locate to the tidal pool at Walpole Bay. But I wanted to go to a different beach, one with a wide horizon and a cradle of chalky cliffs and a cafe, for afterwards.

When I arrive, the sea is distant and has left flat glittering plates of water spread across the undulating sand ridges. A few dog walkers stick to the firm-packed dry sand by the promenade. Out on the horizon, gulls wade. I can see that they are barely immersed. But still, stubborn or optimistic, I trudge toward the sea. The closer I get, the more the water dominates, until my trainers are sinking deep into sloppy grey sand and I realise it’s futile. Even if I did get to the water and manage to swim, I would have nowhere dry to leave my clothes. But rather than retreat, I squelch over to the low sea wall that juts out into what, just hours ago, would have been deep water.

I jump a fast flowing band of water to get to it, then try to get purchase on the rough wet concrete and push myself up. Up I get, eventually, palms scarred from raw-edged barnacles, arms strained with the almighty push. And I walk out and sit at the end and look down, to the toe-deep water, and out to where the gulls wade in the shallows. All of it is the shallows, really. But I have come this far. I have my costume on. So in I go.

The cold wind is a tonic to my skin after days cooped up inside with a cold. I run and I run and the water splashes and I keep running and still the water is not past my shins. The sky is big and the world is a pale tone of blue or grey and when I look back my clothes on the beach seem an age away. Eyes on the horizon I lower my knees, then flop onto my belly. I pull myself over the sand with my arms, channelling my inner swamp monster. The water just meets my shoulders if I really hunker down. Is this swimming? Technically, no. But for the amount of perseverance it took, I’m going to say yes!

The water is cold and clarifying and electric. The colours glitter, the right kind of colours for winter – palest yellow, frosted white – and I feel childlike and joyful as I drag myself across the muddy bottom. It does get vaguely deeper as I hand-wade out. At one point, I even manage to kick a little – just a gentle up and down, nothing so daring as a breaststroke kick. But I am in and I am awake.

I get out and clean my muddy tummy with seawater as best I can. I have a hot drink in the cafe with all my layers on and the day seems brighter despite the onset of dusk. Always, the swim is worth it – even when everything seems to be telling you otherwise!

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