Minecraft wiki entry for Water-powered_boat_transportation, associated text: Signs or buttons are used to block the water at the end of the flow in a channel, forming a gap between one channel and the next. The current from the end of the first channel pushes the boat into the gap, and the second channel's current must pull the boat across the gap, otherwise the boat gets stuck in the gap. The trick to pulling the boat into the next channel is to use only one water source block on each channel, right next to one of the buttons, to create a flow away from the gap.