These were very common in rocky areas.
These were interesting. They were very large, the tube being about 10 cm tall, sticking straight up out of the mud. I saw many tubes, all coated with the same white sponge (tunicate?), but this was the only worm I saw that was out of its tube.
If you've seen the jellies at the Monterey Bay Aquarium, you've seen this guy. This was a smaller, stupider example, who seemed dead set on ramming himself into a clump of eel grass, repeatedly.
I went snorkeling in Tomales Bay seeking this slug ... and I found it! So awesome.
The rock crab on the left seemed like it was mating (it was holding on to another crab with its rear legs). The crab on the right seemed to have a problem with that.