But for a house boat, I think there sort of is an answer to your challenge as different displacements are standardised.

Btw, the standard displacements are a little bit different for a houseboat navigating the european waterways... on one way easier... on an other way much more complicated...
The only thing that counts is: as less as possible into the water (minimal draft) and, what makes it complicated, as less as possible out of the water (clearance under the low bridges).
And this is the only reason why the waterline could change also for a houseboat - there are ballast tanks - full for passing a low bridge (e. g. capestang), and empty if the canal is very shallow (e. g. nivernais)