Watendlath Tarn shines like a burnished mirror. Perfect reflections of the surrounding hills and a Chelsea blue sky are disrupted only by the occasional splash of mallards and greylag geese and jumping trout. Black buzzer flies (chironomids or non-biting midges) on the surface are hatching from the tarn bed.

I think of Judith Paris, the historical novel by Hugh Walpole, which was a bestseller in the 1930s, though little read these days. It is partly set in revolutionary Paris and partly in Watendlath, with tales of passion and murder played out against vivid descriptions of the Cumbrian countryside.

You reach the hamlet by a bottleneck road that corkscrews its way up from Ashness pier on Derwentwater to where the National Trust tea-room sells fishing tickets.

“Watch where you put your shoes,” calls out Stan Edmondson, the proprietor of the Watendlath fishery, as I cross the shore to take some photographs. Just in time, I look down to avoid treading in a patch of olive-green duck shit. He is busy painting one of the rowing boats he rents out to the anglers who come to cast for the brown and rainbow trout that stock the tarn.

The menacing silhouette of a soot-black cormorant perches on a fence post, perusing the scene. As I approach, the long-necked bird opens its hook-tipped bill and utters a harsh croak. Then, flapping its heavy wings like a schoolmaster of bygone days shaking chalk from his gown, it flies away.

On the shore, a scuba diver is assembling his kit. I mention other west Cumbrian divers telling me stories of coming across cormorants while they were plumbing the depths. “Me too,” this one says. “I was once diving 70ft down in this tarn and met one approaching head-on, a bit like a hunter-killer submarine.” He reckoned that the bird might have been flying overhead when it spotted bubbles from his regulator rising to the surface, imagined they were from a shoal of fish and dived to grab one. “I was so spooked I dropped my torch,” he said. “The cormorant sheered away too, just as flait [scared] as me.”