When the boat is placed between two river banks, the pigman follower tends to choose the land path, even though both it and my character were on the boat.
Edit: This problem is kind of general. If your character stands too close to one side and you click on another side, your character will choose the further way too.
1. Put a boat between two river banks. Jump on the boat with the following pigman. Jump to another side. 2. See the additional description.