Mega house boat... kinda

I don't believe that's true -- you're simply limited in your selection of geometry.

 

Somewhat. Even the big supertankers are designed to flex against the ocean - but HDPE would never make it as you say.

I reckon your best bet is an old supertanker, ala waterworld.

But then you are going to be shocked how much interference you will get from official bodies.

You would have to put out to international, unprotected waters - and then you would be in real trouble with weather, etc

 

There are structures which will distribute and absorb loads -- but to make them functional in an unstable environment is another problem. A combination of ballast and buoyancy between two similar structures vertically arranged has been proposed.