Best advice: Dive dry.
Plan B: 7mm with 3mm vest, hood, 3-5mm gloves.
Plan C: 7mm suit, hood and gloves.
Diving: If you're advanced and in San Diego, Wreck Alley is a great time. Vis can be very low and currents can change rather quickly. Still, the Yukon is a great dive. Marissa and Lois Ann are great boats for this area.
Long Beach/San Pedro/LA area, the Sundiver Express and Magician are GREAT boats!! Both usually find excellent sites and never rush you.
The Catalina Express can take you to the island to dive Casino Point dive park. Good times!! Fill station right at the park on weekends, a cafe is steps away. Not much shade, though. Steps go right into the water for easy entry/exit. Underwater there's a few artificial reef sites (sail boat, glass bottom boat, etc). The kelp and marine life are abundant here usually. You can go out at 6:15a and be back by 5p doing 3-4 dives and eating after while still on island. Or you can just stay the night but not much going on after 8-9p during the week.
If you're beach diving, La Jolla Cove and Shaw's Cove (Laguna Beach) are pretty popular. Surf conditions at Shaw's can get prohibitive at times. I don't do much shore diving so I'll leave others to make much better suggestions on this one.