As everyone knows, no romantic day out can be complete without flowers, chocolate and champagne. So when I came up with the idea of a perfect loving day out, albeit one relatively free from these clichéd elements, Sophie's reaction seemed ungrateful, even rather churlish.

"Mackerel fishing?! No way."

My fishing boat-owner friend, Sean, was nonplussed: "And there's loads of cod around too."

I went back to the drawing board. Sophie outlined her idea of the essential ingredients: a large well-laden picnic hamper, good weather, "maybe a rug on the grassy bank of an idyllic river?"

It was then that the idea sprang into my mind. Having just got hold of Daniel Start's wonderful guide to wild swimming places around Britain, I'd been itching to get cracking. Here was the perfect opportunity.

"We'll do it," I agreed. "Everything just like you say … with a few added extras too."

A couple of days before the appointed afternoon, I took her to get kitted out. "A wetsuit? Why do I need one of those?"

But, really, there is nothing as romantic on this earth as helping your beloved into her first neoprene one-piece. We bought gloves – "I look like the creature from the Black Lagoon" – and shoes. I dug out an old dry bag from the attic – "This is the picnic-hamper."

The first stop on our romantic day was to drive, in convoy, to the picnic site, a lovely grassy spot near the sparkling waters of the river Vyrnwy in mid-Wales – not, as it happened, one of the guide book's top spots but a local recommendation.

Leaving one car by our get-out spot, we went on to where we would enter the water and I explained the idea to Sophie. "It's a new sport – I'm going to call it 'river running'. We float our lunch along with us and drift downstream. It's about four miles. We can stop and have a sandwich en route. Romantic days out will never be the same again."

To give Sophie credit she didn't hesitate and was into the water straight away with a few mild screams. "It's going down my back!"

We scrambled over a low waterfall and got into the stream, splashing and swimming. A startled heron flapped away. The gorge narrowed and trees reached over the top. We slid over a shoot and into a foaming pool roaring with golden peaty bubbles.

Sophie was laughing. "Champagne! It's lovely."

We did a long curving run of dark silent water. The river had low rocky shelves on either side, just inches below the surface, but there was a deep narrow central chute. Something splashed ahead, and again.

"You stay close to me or I will get out on that bank and refuse to move," warned Sophie, so we went along close together.

The river had narrowed again and was quite powerful in places. White-breasted dippers and grey wagtails flitted ahead. We were getting bumped a lot now: little bashes against boulders. I went flying over a metre-high waterfall and Sophie came after me, but a little to the left.

I went under, feeling the tickle of the bubbles and that special streak of freezing cold down my spine as a fresh gush of water shot down my neck. When I came up, Sophie was still on the fall, her face grimacing. "Help! I'm stuck!"

I managed to get back against the current and support her against the force of water. "Are you rescuing me, brave sir knight? Now, that's true romance."

Her trapped foot came free and we both tumbled into the pool of golden bubbles, coming up laughing. There were a few more rapids and falls, which we took more carefully, but one of them was so much fun we had to do it twice. After an hour we were into longer, wider stretches where we had to wade in dark chocolate pools, then take to the banks. We pushed through head-high bracken, emerging in meadows filled with harebells and yellow vetches. "Flowers," I said pointedly. I opened the dry bag to discover that it had leaked, ruining the food I'd brought. Nothing, however, could stop Sophie now. She was enjoying the day far more than I had ever expected.

"I love this feeling of being a tough-skinned amphibian," she shouted, running through a clump of nettles and back into the water. "Super-powered – indomitable!"

It took two and a half hours in the end to reach the get-out point. The grass felt warm, and fortunately there was a lavish picnic in the boot of our back-up vehicle: best of all, hot coffee and dry clothes. We lay there, shoulder to shoulder, on a blanket in a patch of sunshine, feeling the delicious sleepy buzz that cold water produces.

Was it romantic? The water was a bit too cold on the day, but the picnic had its moments. Was it fun? Absolutely.

Give it a go. Take local advice. Check the hills above for rain. Wear plenty of padding, especially if there's rocks, and don't go for long – the cold soon kicks in. Above all, get a dry bag that works. On the other hand, you could try the mackerel fishing.

Wild Swim: River, Lake, Lido & Sea by Kate Rew is available for £9.99 (RRP £12.99) with free UK p&p from the Guardian Bookshop.