The best time to dive Northern California is winter, between storms. The water is actually warmer for the most part and the vis is typically much better.
In the springtime, the offshore wind comes up and blows away the surface. This brings cold upwellings full of nutrients from the Monterey Canyon off Carmel. Sometimes it gets down to the mid 40's, and when the Sun hits all that algae and plant plankton, the water turns green. The plus side is that the Sun also brings the Kelp. There really isn't a season, things just change. We pretty much go down every weekend if it's diveable, then jump in and see. If you don't like the conditions when you get there, wait 5 minutes and they'll change.
SoCal tends to get rough in winter, but most of the Channel Island winter trips are for the hunters. Lobster season is shallow diving in heavy kelp with lots of surge, but photogs often go too and conditions can be surprisingly good sometimes. I always do my Channel Islands trip in early August and water conditions are generally excellent, with warm surface temps, up to 60f the rule.
I used to dive year round in a seven mil wet suit, though I've gone over to the dark side now (with no regrets except the extra weight I have to carry). I didn't get that cold on dives in the wet suit, but in the winter the surface interval on cold windy days can be pretty brutal.