Use an ISBU Shipping Container as the “core structure.” I’ve often thought of doing exactly this, using a pair of Shipping Containers as the “body” of the boat.

A trio of high-flotation pontoons under (perhaps constructed by salvaging fuel tanks from aircraft and filling them up with foam), shooting some fiberglass over the bottom of the “boxes” (bolted, filled, and welded together) would even form the hull.

I’m not sure about doing this as an Ocean vessel, but for a lake or river, tucked away miles from everyone? It’d be wonderful! Talk about a heavy-duty houseboat! And you could do it… cheap, cheap, cheap.