I'm not an expert on the subject but I have taken several courses on early HG communities on the Pacific Coast. The short answer is that we don't know a whole lot since all the archaeological evidence from these people is now destroyed or at least submerged and impossible to find. We know that the Pacific Coast was a probable migration route for the earliest people in the New World and that people traveled and settled the Pacific Coast from BC all the way to South America. Hopefully someone else will have a more decisive answer than me.

We know nothing. No evidence has been dredged up or discovered via underwater archaeology. I think most North American archaeologists believe it should be there, but no data is no data.

I have been curious about his subject for a few weeks. I found this article (PDF) a good introduction to the techniques used to learn about the submerged coastal plains and has many references to published articles on the subject: Prehistoric Archaeology Underwater: A Nascent Subdiscipline Critical to Understanding Early Coastal Occupations and Migration Routes