A ferry is a transportation option that does not require a fee. You can use it once you have unlocked a route.
You will need to unlock the ferry landing on both sides before you can use it. You can unlock the ferry by riding to the ferry landing and clicking on it. When you've found docks that connect to one another, you can use the ferry to travel faster over long distances.
You need to be a Star Rider to use the ferry and keep in mind that it can take a minute for the ferries to become active after you log into the game.