This game also has water-based physics. By constructing a raft in game, and then loading it with objects that have rather large weight, we can see for ourselves what effect buoyancy has in-game.
As the boat was slowly sinking in the last picture, the arrows have been drawn accordingly. Sadly, although the objects possess mass, it is beyond me to know how it can be measured. The moment the planks the objects were sitting on touched the water, however, the extra upthrust provided proved sufficent to level off the raft’s position