I lived in one in Seattle back in the '50s. They are built on barges though. The problem with them is not the cost of the boat, but rather the infrastructure and location for them. If they are really live aboard there needs to be connections to shore side sewer, water and power. On a mooring they would never be more than "cottages" because of those limitations. Furthermore, I am willing to bet that if a lot of those showed up in a harbor on moorings, there would be a new ordinance in place pretty quickly banning or at least limiting them. Afterall moorings in Maine are for pretty sailboats and picturesque lobsterboats

As longer term housing they only make sense if they are on a dock with the service connections mentioned above. Because of the cost of dock space, the cheap "shanty boat" will quickly be replaced by the much more expensive houseboat. Take a look at some of the houseboats for sale in Seattle and you will see what I am talking about. Those things are not inexpensive. In fact, houseboats often cost more than houses. Furthermore, space for houseboats tends to be very limited since they take up space that could be occupied by boats. Basically, I see them as a limited niche for the wealthy