Minecraft wiki entry for Water-powered_boat_transportation, associated text: In the third example, you can have another straight flow only in the same direction; you cannot have a new straight flow perpendicular to a previous straight flow, or the boat cannot be pulled across the gap. The straight flow must end with a gap across it, and if the next flow is a turn, it must have a diagonal flow to pull the boat across the gap properly. Otherwise the straight flow can be continued with another straight flow.