I have lived here in San Diego for 5 years now and finally figured out how to dress for this climate. Ironically, I am moving to Seattle next month and now I have to figure out how to dress all over again!
How to dress for San Diego....what I have noticed is that people will wear spring-y weight clothes all year long but the COLORS are what changes. For example, you may wear some light cotton pants in summer and they will be beige or white but the light cotton pants will be dark brown or black in the "winter". It definitely looks out of place to wear your tropical print shirt in the winter...even though it is 75 degrees! And yes, Hawaiian print shirts are absolutely acceptable here. Capri pants are worn year round - light ones in the summer, dark in the winter (sorry, Angie!).
Getting through the cold snaps is done with layers since no-one wears heavy coats - ever! I keep thinking a trench coat would be an ideal way to handle the periodic chilly days but I can't recall ever seeing even one! Really the frame of mind is that it is warm or is about to be warm. So don't bother with a coat.
I live inland San Diego and we experience a much hotter climate than the coastal areas. Hot weather gear is built around shorts, tank tops and sandals. Although this year, I have seen a major surge in sundresses and skirts. And walking shorts! Lots and lots of walking shorts. I'm hoping this means the end of those atrocious capri pants.
I will acknowledge that San Diego has a definite "beach" vibe to the clothing. But I have to say....I still think its inappropriate to wear your skimpy bikini covered only by a sheer swimsuit cover-up to a restaurant when you are not RIGHT ON THE BEACH. I see this all the time (insert throwing-up icon here). And I have seen more cleavage and butt cracks than I care to remember. Last month I was having a nice dinner at a sushi restaurant and a fellow diner was wearing a dress so low cut I was certain I would get some nipple viewing if she leaned the wrong way. Sexy or vulgar...you decide. OK...that turned into quite the rant!
So I hope that answers some of your questions. Basically, winter is here when everyone starts wearing their darker colored clothes! More a nod to the season than actual necessity.