You can dive off Catalina Island and enjoy the kelp-fields and the seals. Top day out mate. I did it in 95 when I lived in LA.
Closer up the coast, you've got Long Beach, which is also a Naval Dock Yard, so you'll meet another kind of SEAL, if you catch my drift. Sea-life plentiful (if you're lucky you'll see Sea Otters) and some interesting rock-clusters and crevices in which to have a mooch.
Oh, and if you see David Hasselhoff, please smack him round the head for me, would you?
According to an article in Undercurrent (American divers' consumer mag - no advertising) in August 2001, there are two good boats that do nice 3-4-dive single-day trips as well as overnighters from Ventura (north of LA) to the Chanel Islands: The Peace and the Spectre. The prices are reasonable and you can sleep aboard at no extra charge. They are not luxury liveaboards but have what you need (including hot showers) and what can you expect for free? The marina at Ventura has lots of shops and restaurants. The telephone mumber to the Peace is (805) 984-2025 and to the Spectre (805) 483-6612 or both boats can be booked through http://www.takemetoo.com. or through Ventura Dive & Sport (805) 650-6500. Hope this helps.
The two places I want to dive up that neck of the Pacific US coast are the Channel Islands and in and around the islets of Alaska - viz about a mile and tons of sea-life. By all accounts the wrecks aren't too shabby either.