Hi! I lived in San Diego forever! Yes, you can swim on So Cal beaches. August is a nice time of year. The June Gloom should be gone and the water may have warmed up to the high 60's or even the low 70's. If you head all the way to San Diego, I suggest Coronado City Beach. Its a nice south facing beach so you wont get hammered by surf. The Navy Seals run by and its super clean. The dolphins swim in close and give an afternoon show. Farther north, Delmar and Encinitas "Moonlight Beach," are nice as well. In Orange County, San Clemente is a great swim beach. I've done a lot of swimming off of that beach, way out beyond the breakers. Its clean and safe as well. In August, they hold their Annual Neptune Festival. Great Sandcastle competition. The resturaunt on the peir has an oyster bar that sits out over the water and is delicious! I don't swim beaches north of San Clemente anymore, although I did as a child. Too cold, to dirty, to shady.
I hope this helps and I hope you enjoy your trip! Please feel free to hit me up if you need San Diego recommendations should your travels take you that way!