My usual summer swim spot is on the lazy, green Yorkshire Derwent. But after school on the hottest Friday of the year, we fancy a change and head for the coast with friends. We love the beach at Hunmanby Gap for its sand, sea, views of the seabird-stacked Bempton cliffs and the almost complete lack of commodification – even the delightfully ramshackle clifftop cafe closes early.

The kids spend hours enacting splashy paddleboard piracy. As the sun sinks, I swim past them, and past gentle breakers into a perfect sea of soft, blue taffeta ripples. But 40 metres out, the surface texture changes from satiny to fuzzy. At first I think it’s floating seaweed, but my husband has a better view from his paddleboard. “Urrgh! Sewage slick, go back!”

There’s been no rain and we checked the discharge map, but despite laws in place to protect us and the environment, and real-time data beamed to my phone, I’ve still ended up swimming in illegal filth from a green-tick beach.

It’s a familiar story, one in which, extraordinarily, our water companies are being investigated for corporate criminality. I thought I couldn’t be more angry, but now there’s a new twist, as some companies appear to be adopting new tactics that subject swimmers to more exclusion, persecution and misinformation. Bristol Water recently closed access to a popular reservoir on hot days and Severn Trent Water is threatening swimmers at Bartley reservoir with costly legal action for an unauthorised dip while permitting other water sports.

As usual, they cite safety. But while every drowning in this sort of setting is a tragedy, cycling, running and horse riding all have far higher fatality rates. The number of experienced cold-water swimmers who get into trouble is extremely small, and the health benefits mean we’re much better off doing it than not. Water companies could do far more to keep us safe by cleaning up their act and promoting safe swimming at sites including reservoirs, as happens in France and Scotland.

Perhaps we shouldn’t be surprised that they dislike us. When the CEO of Water UK linked the awareness of the sewage scandal to the rise in wild swimming, there was more than a hint of a Scooby-Doo subtext: “We’d have gotten away with it too, if it wasn’t for those meddling kids.”