For swimming in lakes, there are two groups of lakes, both 20 minutes from Chamonix, which offer a variety of outdoor sports as well as great barbecue sites at night - completely hassle-free.
There are rules on what you can do and where at each lake, but just do as the locals do. Lakes or areas of lakes are designated for Windsurfing, Fishing or Swimming.
Car parking is heavily police-patrolled so theft is not a problem.
How to get here? Take the motorway, direction Geneva. Come off at Le Fayet, turn left at the lights and drive towards Sallanches. 2km after Intermarche there is a signposted right turn for the lake; before you get to Sallanches. Paid parking.
How to get here? Continue into Sallanches, turn right at the lights in the centre of town. Go straight on, then cross the river Arve (over a high bridge) and turn left. Signposted St Martin and the "lacs des Ilettes".