BURGHAUSEN, Germany — Burghausen Castle, perched on a hill high above this scenic Bavarian town, was still bathed in a white morning fog when the first swimmers of the day approached the Wöhrsee, a sickle-shaped lake that curls along the medieval fortification’s western flank.
At the entrance to the swimming lanes at the Wöhrsee, a lake in Burghausen, a sign indicated the water temperature: 3.8 degrees Celsius, or 39 degrees Fahrenheit.