Serpents do not voluntarily leave water. If a viking drags one ashore, the serpent will move back to the water by the most direct route as quickly as it can.

Dry land structures not within horizontal melee range of water are safe from serpent attacks.
Boats and docks are another matter. In my experience, the most usual reason a serpent would show up at a base on the shore is because a viking lead the serpent there.