Coronado is a city just west of downtown San Diego...and although it has a beach that is consistently rated as one of the nation's best, it is not really known for its diving (this beach is also used by Navy SEALs for training. You might see them out there).
Los Coronados Islands (which can be seen from the city of Coronado) are just southwest of San Diego in Mexican waters, usually accessed by the Humbolt (Waterhorse Charters) dive boat as the Mexican government has imposed new licensing regulations that most operators don't want to deal with. (And I believe the Yellow Charter Boat is also still going out there).
For me, Kelp is the most beautiful thing in the ocean. I think its what makes California diving so enjoyable. I'd say the major areas for Kelp diving in San Diego are Point Loma and La Jolla. Dive boats love to go to Point Loma. They're less inclined to go to La Jolla since the underwater topography is less interesting (or so I've heard since no boat has ever taken me there).
In San Diego, you can also do a shore dive in La Jolla and access the kelp beds that way. If I were you I'd definitely do La Jolla Cove (not La Jolla Shores. There is not one strand of kelp at La Jolla Shores) for a kelp dive, for three reasons: 1) It's protected from shore break 2) Is a very popular dive and you'd hit one of the two main shore dive spots in San Diego (the Shores being the other) and 3) Its a protected underwater park and so there's more life than you might otherwise find.
Outside of San Diego, the Channel Islands are considered superior dive locations to the mainland, with San Clemente Island generally standing out as the apex location for kelp diving. There are a few boats that will go there out of San Diego and a few more out of Long Beach/San Pedro. If you're interested in this option just ask and some of us can
point you to the boats. You're typically looking at $175 - $400 per trip depending on the number of days.
There are also dive sites in Orange County and Los Angeles such as Laguna Beach and Pacific Pallisades that I'm less familiar with. I'm sure others here will chime in w. more information about those.
And a couple of videos to whet your appetite:
VIDEO1 VIDEO2