This is not an answer, but a means thereto that I've yet to try. A colleague of mine has an instrument that could survey the shape of a boat hull out of the water with a high degree of accuracy in a few minutes. (Do not try this at home; the instrument and software would set you back more than the price of a new Catalina 34). Software can easily turn shape into volume, and produce a table of volumes for elevations near the usual water line. If a particular hull were marked at short vertical intervals above and helow its usual water line elevation, you could tell its displacement at any time by observing the elevation of the waterline on the boat.
I've thought about doing the same thing using architectural lines, but I suspect that the accuracy is not sufficient to give meaningful results over the range of interest. And I've always wondered how well molds match the designer's lines....