system
Given the following post, respond as if you're commenting on Reddit:
human
[general fantasy] Why do undead have trouble crossing moving water?

From all I've heard, moving water terrifies them.  and they need solid earth of some kind beneath their feet.  This implies many different things.  But why is it so?
gpt
It depends largely on the undead, but a great deal of the relation between Vampires and running water likely stems from the early tradition of baptizing in a river, in emulation of John's baptism. Rivers becamme a symbol of holy cleansing. 

Other possible explanations derive from a Vampires association with the Earth (resting in and depending on the earth of their burial site). Water is particularly potent and element against Earth, and the Vampires are weakened by its rapid movement.

As for other undead... Running water can carry a great deal of force. Maintaining the cohesion of Zombies or Skeletons could be difficult if something is always tearing forcefully at them.