When I was working as a modelmaker every model I ever worked on had a "Waterline" as datum. Note this was in the "Aerospace" industry, not a single one of them was designed to go in the water. I have no historical reference, but always assumed this "Started" in the shipbuilding industry & originally meant just what Jimmy said, IE all dimensions were referenced from where it sat in the water. The fact that at some later point some designers purposely design a ship to sit different in the water is really immaterial, it was really just a design reference anyway. I have no idea why the gun industry chose table insted of staying with line, perhaps because it was in fact an actual flat on the gun & not just an imaginary line. Everyone has there hang-ups on some terminology I suppose, I try to simply accept what the actual makers use. I still note they "Don't" refer to cutting off a barrel breech section of brazed parts & inserting new bbl tubes as "Mono-blocking", they call it sleeving.
Had to voice my pet peeve!!
