There are great outdoor swimming opportunities available when the weather is warm and the snow-fed lakes have had a chance to warm up.  The Lake Tahoe areas have beach access while the smaller lakes have access over the rocks on the shore.

D.L. Blss State Park has a swimming beach.  It is down the west side of the Lake on the way to Emerald Bay.

Tahoe City

There's a small section of beach at the lake access area in Tahoe City.  It is a pebble beach.  The water is very shallow for a long distance into the lake.  It's a nice complement to the play structures nearby for younger children.
Lake Estelle

Lake Estelle is suitable for swimming in late summer when the water has had a chance to warm up following the snow melt.  It is located in Alpine Bowl uphill from the top of the Kangaroo Lift.  Walk to the top of the lift and look directly to your right to find the trail.
Five Lakes

The larger lakes are suitable for swimming.  The first lake encountered coming up the trail is the largest and probably the best opportunity for swimming.  The furthest lake (Long Lake) is large enough too.
Cascade Lakes

You can swim in Kidd Lake and Long Lake among the Cascade Lakes near the Soda Springs ski area.  The trailhead is located behind the Royal Gorge X-C ski area on old Route 40 near I-80 (Soda Springs exit).
Donner Lake

There is a beach with a swim area at Donner Lake.  They have showers and rest rooms.  There is an admission charge.
Serene Lake

Serene Lake is fully developed as a recreation area with a beach, rental boats and services.  It is directly behind the Soda Springs ski area.
Kings Beach is a larger beach at the north end of the Lake past Tahoe City.
William B Layton Park

This park has a beach on Lake Tahoe suitable for swimming.  It is just south of Tahoe City on Route 89.