A Ferry Boat can be used to cross small bodies of shallow water (8ish coords max).
This building becomes available after once you have learned Shipwright 0 (Skill).
Built Outside (Right-click on the ground: Projects > Travel).
Build the ferry along a shoreline, then Ride the ferry to travel across the water.
When building it, changing the direction of the ferry is purely for aesthetic purposes - the path taken by the ferry appears determined by the landscape (roughly speaking, the nearest shore at 90 degrees to the lay of the land where it's built).
It can be disassembled for a full refund (don't tear it down) with all the parts going directly into your inventory (to be rebuilt elsewhere).