It describes the river analogy only to the point of ignoring everything else, assuming no FOC and no forward velocity/momentum. Your river analogy didn't consider the force of current/water over the area of the vessel to weight ratio in figuring how fast each would be brought up to speed. It's concievable the drum might be brought up to speed faster, since the cork has so little area exposed to the current, just bouncing long on top.

If one did the drop test from 10 meters then repeated from 50 meters, based on drift alone you'd probably come to 2 different conclusions. That would not be explainable by inertia, FOC, or arrow velocity/momentum alone.