Minecraft wiki entry for Water-powered_boat_transportation, associated text: After the final dirt block is removed, the two-block-wide space in front of the step has a void space (occupied by a button) and a still-water block (where the dirt used to be). Another void space is upstream ("downhill" direction) one block diagonally from the void space at the step. Otherwise the channel is completely filled with flowing water. Because the boat is bigger than one block, it does not get stuck in either void space. The current pushes the boat diagonally into the still-water block at the bottom of the step, at which point the boat floats up into the flowing water above, and the boat continues on its way.