I've seen them many times in San diego (where I live). Octopus don't like to come out and walk around much during the day because it makes them vulnerable. The only times I seen them out in the open is at night while scuba diving, and that is hit or miss.
Do you want to pasively observe one, or catch one and look at it (and then release it)? For either approach I would suggest going to a place that is know for good tidepools (Laguna beach?) at really low tide. There will be really low tides (less then -0.5 ft) coming up during daylight hours on October 14 - 17. Such low tides haven't happened in about three months, so don't miss it! To passively observe an octoups, head down to the tidepools about 45 minutes before a low tide (lower than -0.5 ft) and look for rock overhangs that are a few inches submerged, and that provide a deep dark crevice back under them that would hide an octopus. Then, pull a few muscles off of the exposed rocks, open them (with a dive knife, a rock, whatever) and place a half shell fill of yummy looking muscle on the sand (submerged) in front of each of the little overhang/caves that you identified. Wait ten minutes and then check each one to fine out which muscles have disapeared. Then put another muscle half shell there, and watch it. Within a few minutes you'll see an octoups arm reach out for the open muscle. This will tend to be a fully grown octopus if the crevice is large enough for that. Now that you know where he is, you could either be content to just watch his arm grab food, or if you have an underwater flashlight, and a diving mask (and willing ness to get all wet) you could duck your head down and look into that little cave.
Don't do this in a "Marine Protected Area" (MPA) where the muscles are protected, but in any normal area, you are allowed to take up to 10 lbs of muscles per day with a fishing license. If you don't have a fishing license then you are technically in violation, but I doubt that you would get in trouble, and you couldn't possibly hurt the muscle population. The MPA's are listed in the back of the California Dept of Fish and Game - Ocean fishing regulations manual.
To catch an octopus, you could use the above method to try to lure the octopus far enough out from his cave that you could grab him, but I doubt that would work. A more effective aproach might be to to look around for flat rocks in a foot or so of nice still water (also at low tide) and tip them up on edge hoping to surprise an octopus that is hiding under the rock. Tip it up slowly so that you don't make a cloud of sand, and look for small octopus (big ones live in rocks that are way to big to be tipped up. A surprised octopus will jet away, maybe ink, maybe hold still and try to blend in, or possibly try to crawl under the nearest rock, or the edge of the rock you just tiped. Don't bother with small rocks, less than a foot across. A rock resting on a rock is more likely to have an octopus under it than a rock resting on sand, and look for rocks with lots of stuff growing on them. Also, don't flip the rock all the way over, just tip it up on edge with one hand and grab with the other hand. You don't want to squish all the stuff living on top of the rock by flipping it all the way over. I recommend wearing leather gloves because the rocks could be sharp.
It is REALLY IMPORTANT to tip the rock back down into the same position it started in so that all the things that are living on that rock end up in the same position. Shade, sun, water currents, sand, etc. all cobine to create the exact conditions that each animal on that rock needs to live. If you move that rock to a different position, or leave it flipped over, you will kill almost everything that was living on it. Put it back precisely!
