The lower end of Lake Chelan [1] is lined with sandy beaches just right for swimming in the clean, cool waters. By late summer the water at the lower end of the lake is a bearable 70°F.
Most public parks have popular swimming beaches, including Lake Chelan State Park, Chelan City Park, and the newer parks built by Chelan County, including Manson Bay Park, Old Mill Park, Chelan Riverwalk Park, Chelan Falls Park, and Beebe Bridge Park.
Up-lake beyond Manson and Lake Chelan State Park, the beaches become fewer due to the steep terrain. Swimming is possible in the Stehekin [2] area, but with the water seldom above 50°F, few people are found in the lake by choice. The lake never freezes.
Links:
[1] http://www.moon.com/destinations/washington/north-cascades/chelan/sights/lake-chelan
[2] http://www.moon.com/destinations/washington/north-cascades/chelan/stehekin