Legally USCG documented tonnage is a volume, I did the calcs on my boat to see if the would document it.
Lexicon-wise you can talk about displacement as weight or volume:
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noun: displacement; plural noun: displacements
the moving of something from its place or position.
"vertical displacement of the shoreline"
of someone or something by someone or something else which takes their place.
"males may be able to resist displacement by other males"
the enforced departure of people from their homes, typically because of war, persecution, or natural disaster.
"the displacement of farmers by guerrilla activity"
the amount by which a thing is moved from its normal position.
"a displacement of 6.8 meters along the San Andreas fault"
the occupation by a submerged body or part of a body of a volume which would otherwise be occupied by a fluid.
the amount or weight of fluid that would fill the volume displaced by a floating ship, used as a measure of the ship's size.
"the submarine has a surface displacement of 2,185 tons"
the volume swept by a reciprocating system, as in a pump
"a fixed displacement gear"
the unconscious transfer of an intense emotion from its original object to another one.
"this phobia was linked with the displacement of fear of his father"
the component of an electric
field due to free separated charges, regardless of any polarizing effects.
the vector representing electric
the flux density of electric displacement.