We go to the place where the seals always are, except they aren’t there. We walk back along grass blown cliff tops, looking out over a sweep of sand that runs further than they eye can see, all the way round to St Ives on the headland. Surfers bob in the water, waiting on the waves. The waters rise high but roll back. The tide is coming in but once the water breaks it is shallow. We do the sensible thing and ask the lifeguard how quickly the smaller bay below his hut will be above water. He shrugs casually and says at least 40 minutes. We go down.

Dressed only in towel robes on top of swimming costumes, we have little to deposit on the sand. Even so we make sure to keep our things well clear of the clutches of the sea, propping them on rocks above ground level to be sure. And in we run.

The waves are powerful and the lowness in the water between makes it hard to swim. The sun has disappeared from the sky and though it’s not dark, the light is a pale blue-grey and it settles over the washing water like cloud. We jump into the surf, foam flying, salt spraying, lifting us, carrying us, dropping us.

In both directions, the beach continues. A rocky headland to our right marks only a corner, not an ending, and to the left is open shoreline for miles. The cliffs are golden behind us, crumbly in the evening’s light. The water is moving in fast and it’s lucky we stowed our things above ground level, for all the sand is pools and puddles. So much for 40 minutes. We wade, at times knee deep, back to the landing ramp we came down by. The limpet colonised rocks at its base are now invisible beneath swirling eddies of pulsing seawater and we can’t see the smooth footholds any longer. Still, we clamber up and out, then turn at the top and watch as two surfers, boards strapped to ankles, wade down the now treacherous path. The sea watches, sullen and grey, as we turn away from it and head home, keeping our wet robes on for the journey.

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