Yes…Mammoth Lakes does have a lot of lakes, but not all of them are family friendly places to swim.  Any lake is accessible to tip those toes into and even for that cannonball off a cliff but here are a few places that have nice beaches to spend the day at enjoying every water activity that you can think of.  Before I begin, these are mountain beaches, not ocean beaches.  This means small rocks instead of finely broken down sand.  

In Mammoth, many folks enjoy hanging around Horseshoe Lake at the very end of Lake Mary Road.  This is a beautiful large lake with a trail that encircles its circumference.  The beaches are usually pretty gentle sloping and have good sun exposure but the soil is a bit diluted with Co2 from underground volcanic vents which is why you’ll see many dead trees on it’s edge.  There is no concern for you but If you try to fish, you might wonder why you’re not catching anything.  The next lake is June Lake in…you guessed it…June Lake.  Within a 20 minute drive north you’ll stand in awe at the mountain scape that appears in it’s grandeur above June Lake.  Take the first right and drive through the campground to get down to the beach.  On a nice summer day, this beach will be quite crowded but there is still room in it’s vastness to not be completely immersed in their sun shades and frisbee games.  One of my favorites times to visit June Lake is on a full moon as there is a bon fire area to enjoy a nice social gathering.

TIP: From the parking lot by the Gull Lake campground, take the trail to the left (versus going into the campground).  You’ll find a tree swing that some local put up and fun cliffs to jump off of.