I think it is more accurate to say that the buoyancy characteristics of a board are related to volume distribution.
Any floating mass will displace a volume equal to its mass. Example: An 8kg board will displace 8L of water (1L = 1Kg).
From this you will note that quoting "displacement" is pointless as it is the same as the mass of the board.
The mass of board+rider+rig will tell you how much water you need to displace in order to float - nothing else.
This issue of volume not giving you enough info is the main reason most manufacturers quote "width" as the primary differentiator for boards within a style now. You will also notice that Starboard are also looking at the distribution of thickness as a function of volume this year (Futura etc).