Long Beach, no diving there due to harbor, and foul water over sand bottom.
Palos Verde.... Ahhh, now that is a different story.
Malaga Cove
Flat Rock
Long Point/Old Marineland
White Point
Cabrillo Beach(uh, no thanks, bad water score)/Point Ferman(ok)
Christmas Tree
and more.
http://www.sandeaters.org and then dive sites to see photos of these locations.
Palos Verde Dive sites will require a walk and some climbing down hills (and back up). Many sites are down goat trails or worse.
Long Point, Whites Point (not a great dive IMHO) and Malaga Cove are the easiest access and are good dive spots.
Flat Rock is for the in shape diver only, and I do mean in shape. You should also not have a fear of heights and be very sure footed.
Christmas Tree is reached by climbing down the landslide face. Steep, but not too bad relatively speaking.
http://www.healthebay.org/brc/statemap.asp