Doing some research - the East Bay actually has a ton of swim lakes, which seems like it should be awesome, but all of them seem to be fucking lame like Anza where you're not allowed to swim anywhere in the lake.
The one exception is Del Valle out near Livermore. That actually seems pretty awesome, it's a big lake and you can swim anywhere in it. A few problems though. 1. Motor boats are allowed. 2. It's pretty far from here. 3. It's 92 degrees there today.
One highlight at Tilden - we layed out in a field for a while and watched a feral house cat hunting.