I have done extensive research, and I have not been able to verify that it is illegal to take strawberry anemone (Corynactis Californicus) from California coastal waters. It is generally believed (but not true) that it is illegal to keep an aquarium in California with native California coastal animals in it. I live in California, and I've carefully searched the Dept. of Fish and Game (DFG) regulations (and California penal code) and while some things are illegal, many things, including Strawberry anemone, are legal.
Assuming one has a California fishing license the restrictions are:
1) No collecting in "Marine Protected Areas" (MPAs). There are explicitly listed and defined in the DFG regulations. Most state beachs are NOT MPA, and most areas of the coast are open to collecting.
2) No fish may be taken away from the ocean alive.
3) No invertebrates may be taken from "the intertidal zone" EXCEPT the things listed in the "invertebrates" section of the DFG Regulations. The list include (allows) muscles, clams, octopus, etc. (see the regs for specifics) The list of things you may take from the intertidal zone includes the things that people have traditionally gathered for food.
4) The intertidal zone is defined as an area from the mean high tide line, extending 1000 feet seaward. That will include all tide pools, and even 1000 foot radius around any rocks that are exposed at mean high tide.
5) There are open seasons, size limits and bag limits put on some things (like abalone and lobster), and a very few specific species are illegal to take at all, but there are no restrictions beyond those that I've mentioned.
So, based on #3 above, it is illegal to take any anemones from within 1000 feet of the mean high tide line. That means that it is NOT ILLEGAL to take anemones that are found outside that zone. Generally, Strawberry anemone live deeper than the intertidal zone anyway, so if you are more than 1000 feet off shore you can take anything that is not specifically prohibited, and anemones are NOT prohibited. IF one has a boat, or even a surf board, it is not hard to find rocky areas 1000 feet off shore. If you can SCUBA dive, or even just hold your breath, you can find strawberry anemone, or many other very cool things to keep in a cold water aquarium.
Not everyone will be willing and able to dive for animals 1/4 mile or more off shore, but for those who, there are few legal limits imposed. And there are many things (on the list) that are legal to take from easily accessible tide pools (snails, shrimp, octopus, worms, and crabs for example.) If you are willing to do the swimming, and live withing the rules I've mentioned - go for it!
Now for the disclaimer. I am certain that collecting animals, within the limits I've described, is within the law as it is written. What I don't know is, if the law is enforced as it is written. Given the passion that many people have for nature, it may be the case that some state agencies, and/or judges, will give tickets and impose fines for doing things that are not illegal. I honestly don't know what the common enforcement practice is. As a result, I play it safe by trying to be discrete. I'm a little careful who I tell about my tank, and I keep records about where I collect things, so that I can show, if I ever need to, that I was beyond the 1000 foot limit (where applicable). I don't know where the belief came from that keeping California native marine animals in a home aquarium is illegal. It may just be a myth, it may come from the fact that it really is illegal to SELL wild caught California animals (without a difficult to get and expensive permit). At my most paranoid I worry that maybe people are being routinely cited and fined for doing things that are simply not illegal. Anyway, there may (or may not) be a risk. I choose to keep the animals, keep a low profile, and if I ever have a problem, at least I'll have the letter of the law on my side when I fight it.
