Diving is mainly of 2 types: skin diving with snorkles and scuba diving with tanks.
You can snorkle almost anywhere.
Just keep a distance from others, so nobody steps on you by accident.
The most interesting diving is not on regular beach areas, but rather by rock jetties, piers, and other rocky areas.
Always be careful that waves and water movement don't throw you against the rocks you want to look at.
Serious Divers choose the areas with interesting things to see underwater like the coral at Crystal Cove in Corona Del Mar, and most of Laguna Beach, South Laguna, Capistrano, San Pedro and the Palos Verdes Peninsula to the West, Catalina Island, and from Malibu to Santa Barbara.
Better descriptions of locations can be found at ladiver.com.
Info on Diving Boats can be found at californiadiveboats.com.
Other Diving Resources can be found at Diver.net