I've always found that layers are good when visiting CA. Or living in CA. I remember well bundling up for walking to High School in San Jose much of the year, slowly unlayering as the day went on. But that's near San Francisco, not southern CA.
Our first big family trips were in September and then December. We found the daytimes in December to be much warmer than most of the daytimes in September. But then...in the evening, it got cold fast (I find it gets cold fast in Anaheim, once the sun goes down), and it rained at least one evening. That's the evening my guys wanted to leave for the hotel, I stayed, and I rode Grizzly for the first time! I wore fast-drying pants and had a poncho on, and I was FINE.
I don't wear jeans; haven't done so since I was a teenager and finally realized "these are just NOT comfortable and weigh approximately 800 lbs"...in fact, at the parks, i generally wear knee length, Hanes brand cotton skirts with cotton shorts under, and as long as my top half is covered for warmth or relatively uncovered when it's hot, I'm happy and fine. For feet I either wear my
Crocs Athens or my running shoes, and I change shoes through the day as I need to to stay warm/cool/comfy.
We'll do the water stuff; just gotta make sure you have something to change into, something to protect what you don't want to get wet, or wear something that will dry fast.