Flowing out of Lake Zurich northwest through the city, the Limmat is a giant river that is both navigable by smaller watercraft and swimmable, just be careful where you jump in. From the Quailbrücke to the Lettenkanal swimming is forbidden, but there are plenty of designated spots for taking a dip including the Frauenbad am Stadthausquai and Oberer and Unterer Letten.