It seems that two different issues are being discussed. The comparative weight per length and how it affects a boat's motion is one issue. Seaworthiness relative to weight is another much more complicated equation. Simple displacement has little to do with the sea-kindliness of the boat. It's more a function of design. "Heavier" has some connotations that are misleading, leading one to equate heavier with a more comfortable ride. Boats with the same displacement can have radically different shapes and characteristics.