Yup, that's what I meant although it is not exactly "volume of the hull". The boat density should be something like "density as you would calculate it for any s

Yup, that's what I meant although it is not exactly "volume of the hull". The boat density should be something like "density as you would calculate it for any solid object" plus "all air below the water line". So solid parts always count and air only counts if its below the water line. This way Titanic counts as just a heap of steel (and doesn't float) and Ever Given counts as steel-air mix (and floats).