From my testing, it seems that pufferfishes on top of boats will slowly move in the northwest direction when they are inflated to any level and the world is reloaded. After some time, the pufferfish will clip into the walls on the northwest side. However, when using fences, they never seem to clip into the wall, but will still end up in the northwest corner.  Images show pufferfish before and after reloading the world a few times and inflating it at least once each time.

Additionally, the hitboxes of pufferfish on boats seems to be buggy when the world is reloaded, which might be causing this issue. When reloading the world, it seems like the pufferfish have the hitbox of a inflated pufferfish until inflated and deflated again; when they are pushed against a wall before being inflated and deflated and after the world has been reloaded, they seem to have an invisible barrier stopping them from hitting the wall completely. Even turning hitboxes on doesn't show the barrier stopping them from hitting the wall.