Seattle also has a limited land area, because of the ocean and lakes. There is only one way to get more housing, build up, or build outside the city limits.
Maybe more of those would come in handy here in San Francisco too!
Probably more impactful in terms of population numbers is the similar "apartment block over water" which popped up in Seattle a few decades back and are still around. ~5 stories and over water is a lot more people than a row of houseboats.
[1] https://www.google.com/maps/place/Floating+Homes+Association...
It been a few years since I heard that though, it may be incorrect or outdated.