Porthcurno is a beach like none I’ve seen in England before. Sand like sugar, water turquoise and crystal clear. We get there early and there’s only a couple of other people on the beach, not a single soul in the water. The sand shelves steeply down from the shoreline and the breakers are pushed through quickly. Then you are in deep azure water, water so clear you can see every inch of your body below the surface, can see right down past your toes to the hundreds of tiny granules nestling side by side on the seabed.

It is a sea that begs for diving and looping, kicking down till the water meets way above your head and nothing but endless clarity, impossible blue stretches ahead of you. Reaching arms reflect the calligraphic light waves of sun filtered through a dappled surface. Black-brown rocks frame the bay and I swim along, parallel to the shore, till I’m close enough to see the lichen spotting them with grey. Turning back, I spot a tiny piece of plastic and enclose it in my fist. It occurs to me that it’s only so noticeable because of the startling lack of litter elsewhere on the beach. This place is happily well cared for.

I surge forward with the waves, riding the white water. I plunge head first, again and again, at the bottom, letting the water lift me through the layers of blue, turning full circle to see the fathoms that stretch away from me, before letting the buoyancy pick me back up, gasping, to languish again on the surface and catch my breath for the next time.

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