I love to swim in open water all year round. Before I moved to Cornwall I was swimming in rivers and lakes, although I had to give up the river swimming in autumn once the water levels rose and mud choked the streams. I would watch the river rushing along in full spate after heavy rains, pushing ahead branches and great swathes of farmyard plastic as though furiously trying to cleanse itself, and I would know that the river was not in the mood to welcome me for a while. So I would go to the mirror calm of a lake nearby instead and swim ever decreasing lengths as the temperatures dropped. It is perhaps because I retreated to this quiet, manmade, inland stretch that I never truly realised the impact of the changing of the seasons on swimming in cold water, other than what the falling mercury told me.

“Four degrees today!” we’d yell as we plunged in and checked the thermometer hanging from the pontoon. “Nearly two – look!”

We took huge pleasure in pushing our bodies to these limits, huge pride in what others saw as madness. Or stupidity. It made me feel alive and happy, no matter what else was going on that day.

Now that I swim in winter seas, I know a different kind of swimming. The temperatures never fall to that of inland water, so it is possible to stay immersed for longer and to do more actual swimming rather than simply bobbing about. Nevertheless, the winter sea presents other challenges: even on a relatively calm day, the current is stronger than in the summer months and you can find yourself pushed and pulled around when you least expect it.

Yesterday I went down to the cove and felt that rush of delight I get when I realise that the water is calm enough for me to enter. It is a case of “each day as it comes” at this time of year: you can leave the house on a windless day, the sky blue with tiny scudding clouds and think, “I’ll go for a swim”, but as you walk down through the woods you will already be hearing the sea roaring and the wind whipping itself into a frenzy. It is never possible to predict what the conditions will be like. Yesterday, though, they were perfect.

I lowered myself into the jade-green water. Sunlight danced in a path from the Point, as though laying down a golden trail for me to follow. The cold seized at my chest and the back of my head as I pushed out and I took deep, panting breaths, feeling them catch in my throat, forcing myself to breathe out slowly, not to gasp. Gasping is the most common cause of drowning. I remind myself of this every time I swim in cold water.

I pushed out and out and was suddenly caught by a medium-sized breaker, the type of which in summer I would ride out with pleasure. This was a high spring tide, though, and the water was muscular. I was pushed towards the rocks and had to strengthen and lengthen my stroke, swimming diagonally against the tow. A mild panic spiked through me, but after only three strokes I was out of the worst of it, allowing the current to take me serenely back to shore. The shock was not enough to put me off striking out again and I managed six “lengths” of the little inlet before deciding that, sadly, enough was probably enough for one day.

I went back to my desk and tried to write. This is what I have been doing for months now – trying. My thoughts push and pull me around. I strike out on to the page and find myself pushed back again and again. It is a case of one word forward, three words back. It has been like this for so long I am beginning to think that I will never write another book. When that thought strikes me I am seized with the same spike of panic that shoots through me when the sea pulls me back towards the rocks.

And then I think, if swimming is harder in winter, is it any surprise that writing is too? Especially during a winter in which I am forbidden from going anywhere or meeting anyone. I am trying to write a book that requires me to meet with people to interview them, to go to libraries to do research. Yes, I can phone people and email them and there is always the internet for looking things up. But that would be like going to do lengths in an indoor heated pool, choked with chlorine and lit with a harsh artificial glare, instead of welcoming the sea’s invigorating embrace.

Phoning someone – or worse, Zooming them – is not the same as sitting down at a café table, the windows steaming up and shielding us from the gales and storms as we laugh and tell stories and share thoughts and dreams. Googling information is not the same as going into a reading room that is reverberating with the energy from the silent whirring cogs of a hundred brains, all thinking about and reading about and writing about a hundred different things.

But then swimming in the winter sea is not the same as setting off across the bay in a millpond flat sea, spotting a curious seal who is lazing on his back, or watching as the cormorants take off from a rock to skim the surface near your head. Yet the experience of the water is no less magical in November than it is in May, merely different.

Maybe, then, I should practise some acceptance: I should tell myself that, sure, there are days when the writing is easy and flows as smoothly as warm, sun-gold water, and there are days when the undertow has you in its grasp and the words are flung up high up on to the rocks and out of your reach. The trick is perhaps not to fight it, but to write differently – in a diagonal, accepting the effort involved, taking my time, until the words are ready to ease and carry me back to shore.