Seattle actually has some houseboats [1]. Not sure total numbers, but they are definitely a very small drop in the bucket compared to all other housing forms.

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...

If I'm not mistaken, houseboats in the Seattle area also have property tax tied to them as well. Instead of just a slip rental/lease.

It been a few years since I heard that though, it may be incorrect or outdated.