I found the lake in question. Its Perris Reservior which is east of Los Angeles.
Names of beaches found here:
- Perris Beach
- Moreno Beach
It has the distinctive island poking out in the lake
Exact latitude and longtitude:
33 degrees 51 min 18.00 sec North by 117 degrees 10 min 34.00 second West.
Update: Island in this lake is called Alessandro Island
Sanosuke!