I wear a 7mm suit year round here. I could use my 5mm during the warmer parts of the year, but I love diving SoCal and not feeling the water at all.
In NorCal a 7mm is necessary, as the water temperatures are quite nipply (especially in the spring when upwelling occurs). Depending on where you are diving in California and what time of year, water temps range from the low 40's to somewhere above 65. I say somewhere above 65 because I have never dived in the southern part of the state when the water is warmer than 65.
I grew up in SoCal, but only started diving when I moved to Humboldt for school. Temperatures and visibility are considerably less desirable here than in the south. For me, the cold and poor vis are nothing to complain about. I actually love it. In the beginning, the exposed skin of my face would hurt, and my lips wouldn't work properly after a dive. After a year of diving here, I became acclimated and now I don't even notice the cold on my face. I can even talk normally after 4-6 hours in 50 degree water! The limiting factors to how long I can remain in the water now are my fingers and toes. If they stop working, I need to get out because they are a very important part of safely navigating through the dynamic shore exits and cliff hikes we frequently encounter.
As far as the weight issue, I wear about 18lbs of lead on my waist to offset the buoyancy of the suit (all of which is lead I have found while diving and remade into a bunch of 2-4lb weights :thumbsup2:). This amount of weight makes me neutral at about 23ft. If I know I'll be doing a significant amount of deeper dives, I take off 2-4lbs for safety and ease of ascent. This weight reduction makes me neutral at about 40ft. I have seen some people who wear harnesses while spearing, but I feel that harnesses provide that much more material to get snagged on kelp and potentially cause problems.
Diving in kelp is awesome. You can use it to conceal your approach, you can pick some to accompany your dinner, you can just plain marvel at it. When you are in kelp and the water is clear, the sunlight has a magnificent golden hue.
Back to visibility. In Humboldt and Del Norte counties, it's usually bad. A good day is 8ft vis. It's typically 3-5ft. This is due to many factors: the amount of fine sediment coating the reefs, the number of rivers and streams discharging into the ocean, and the exposed coast that gets a good amount of energy sent down from Alaska and Canada. When it's greater than 8ft, the diving is absolutely breathtaking here.