The signs on the gate at the entrance to the path and along the edge of the reservoir were clear. “No swimming,” they warned, white letters on a red background. On a chill mid-April day in northwest England, with low, gray clouds and rain in the forecast, the signs hardly seemed necessary. But then people began arriving, by the dozens and then the hundreds. Some walked only from nearby Hayfield, while others came by train or bus or foot from many hours away. In a long, trailing line, they tramped up the hill beside the dam and around the shore of the reservoir, slipping in mud and jumping over puddles. Down on the shore, giggling and shrieking people picked their way across slippery rocks. Then, with a great deal of cheering and splashing, they took to the water en masse, fanning out in all directions. Some carried a large banner that read, “The Right to Swim.” More rounds of cheers went up as new waves of swimmers splashed into the water. An older woman wearing a pink floral sw...