My goodness.... You're even worse than I am when it comes to technical specifications........
But you are correct -- a given ship's "weight" if you will, is the amount of sea water it DISPLACES as it bobs about at anchor.
And you're right -- various hull designs contribute to various displacements.
It would be nice if ship displacements were standardized. Heaven forbid there be a common standard.
But I guess, Gross Registered Tonnes (GRT) is a recognized standard. Perhaps if the cruise lines were to stick with that, life would be easier..........