Liveaboards
Some history on this here.
The phenomenon seems to be isolated to a highly-regulated section of Sausalito now.
One article on the end of the Redwood City community can be found here. The area known as "Docktown" may still be alive. I don't know for sure. They were still active at the end of 2006.
I like to think there are still a lot of "informal" boat dwellers around. They aren't parasites on society, even if they don't have legitimate abodes. Even the poorest ones I've seen were hobos, not bums.