It might not be safe to swim in the lakes or rivers that flow through most European big cities. However, in Switzerland, you are welcome to enjoy such things as you like. Try the Bains des Pâquis in Geneva—an open air public swimming spot right in the centre of town. Built in the 1930s, it is still a favorite place for locals during the summer to socialize, swim, sunbathe, and have a meal with a beautiful view. If you are not comfortable swimming in the lake, check out Bellerive Plage Lausanne. This 1930s open-air pool and leisure centre is located close to the Lake Geneva bank and is extremely popular during summer.
Lake Zurich and its inflowing rivers also become a beach spot during the summer with its famous "Badis." Visit Zurich Flussbad Oberer Letten, a hang-out for Zurich hipsters at the Limmat River or the more quiet Seebad Enge. Swimming, beach volleyball, and many more activities are available during summer—like renting paddle boats and exploring the city from an unusual angle.