I combed through the regulations and statutes before I caught my bimac, and I came to the same conclusion as this thread. I have found, as was found in the thread, that law enforcement people are often incorrect when asked about laws that seldom show up in practice, and tend to say "nope that's illegal" just to be on the safe side (or to discourage activities they wish were illegal). I only use the written law to determine what is legal and what is not. Having done my homework concerning California DFG regs, I'd like to clarify and add a couple of things:
1) While the sale, barter or trade, of octopus is prohibited, it is perfectly legal to give one away for free. This law applies to everything you get with a fishing license, not just octopus, and it is a time honored common practice for fishermen to give some of their catch away to neighbors, friends, family, or whoever. If you make a business out of it, or even trade, like the tropical reef guys trade coral frags, then you are in trouble, but I can catch an octopus, and give it to anybody I want, as long as I don't get anything in return except gratitude.
2) Not only is it legal to take everything except fish away from the ocean alive, but there is a good reason for it. Lobster, muscles, and other shell fish quickly become toxic after they die, which is why it is always recommended to cook them alive. Any law to the contrary would result in food poisoning.
3) There is a specific list in the regulations of what inverts may be taken from "the intertidal zone" (tide pools) which includes octopus, but also most of the other things that you may want from your tank, or use as fresh octo food. All other inverts are protected in tide pools. See section 29.05 (b):
"...tidal invertebrates may not be taken in any tide pool or other areas
between the high tide mark (defined as Mean Higher High Tide) and 1,000 feet seaward and lateral to the low tide mark (defined as Mean Lower Low Water) except as follows:
(1) Except where prohibited within state marine reserves, state marine parks, state marine conservation areas, or other special closures only the following may be taken: red abalone, limpets, moon snails, turban snails, chiones, clams, cockles, mussels, rock scallops, native oysters, octopuses, squid, crabs, lobsters, shrimp, sand dollars, sea urchins and worms..."
There are size and bag limit restriction on some things (lobster, abalone, etc) but all those things may be taken from tide pools.
4) The above regulation: "you can take nothing except..." only applies to tide pools, and 1000 feet seaward. Outside of those areas, the rules are inverted: "you can take ANYTHING except...". That means that if you can find a rocky area that is more than 1000 feet away from any land or rock that is above water at low tide, then you can collect almost anything from it, even things that are not on the list above. That means you can take anemones (like strawberry anemone), sponges, gorgonians, sea stars, etc. You probably want to document where you collected these things, in case you ever need to explain that you didn't get them from the tidepools, but it is legal.
5) you are allowed to take at least 25 lbs of rocks, per person, per day in California, without a permit (except of course from a specifically protected area) That includes rocks from the beach, although any animals or plants growing on the rock are subject to the restrictions I've mentioned above). So it is legal to take cool looking rocks from the beach for your tank. You should look up the actual statute (written for rockhounds, not fishermen) and carry it around too. It's a little more complicated than I've stated here, but you can certainly grab a few bare rocks if you're not in a protected area.
6) You can get in trouble even if you are within the law. Many people feel certain that it is illegal to collect from state parts (wrong) or illegal to collect for your aquarium (wrong), and you may encounter these people when you are at the beach collecting. Some state park rangers, who have the power to give you a ticket, are in this group, so I always carry a copy of the regulations with section 29.05 highlighted, so I can show them, if confronted, that I'm not breaking the law. Additionally, there are a lot of self-righteous eco-zealots in California who will give you vicious verbal abuse and threaten to call the police if you don't stop what you are doing immediately. They might be shut up when shown the law, but more likely they will simply switch from threats of calling the cops to vehement condemnation. Be mentally prepared to be mistreated by these fanatics. I have to deal with one about 25% of the times I collect at the beach.