Welcome to Scubaboard!
All year diving is the rule around here. While the summer time can bring green water and bad visibility, that's not always the case. Some of the recent dives around the Channel Islands have been spectacular with blue water and extremely good visibility. It's never a dull moment and, I think, some of the world's best diving.
You can dive all year in a 7mm - many of us do it (though I am an admitted drysuit fanatic). Sometime around June, a lot of folks drop the gloves and the hood and even a few others drop to a 5mm. In my experience, water temps start to cool down around November and usually get to their lows around February. It's around April and May that things start warming up.
For a novice diver, I would highly recommend Casino Point over on Catalina. You can hire a local DM to show you around and give you the grand tour. It's where many classes are taught and is a lovely dive with just about everything - kelp, fish, etc. If you're not used to getting in and out of surf, it's an easy entry via stairs.