Lake Annecy, in the old city

Sure. And not only swimm, but you can also cross the lake in a canoe, paddle boat or cruise boat...Lake Annecy is located in the Vieille Ville, the Old City, and it's surrounded by parks and greenish areas, where the locals come weekly to picnic and sunbathe. But things can get even better: 1 km East from Champ de Mars, there's Beach Annecy, open from June to September, and 20 minutes away walking south, you'll find Plage des Marquisats. For the price of 3 euros you can also gain access to Plage Imperial, with changing rooms. Prepare your sun lotion :)