I don't really think that your going to want zombie fish in your moat when a major hoard attack. If anything, they could hinder you from getting away if your base is overrun. Obviously they won't attack other zombies.
Not a great idea for anything else ether. PVP players are too smart to dip there toes into that and will find a way around.
Bandits are unheard of and not even an issue at this point.
On the other hand having fish would be a great idea. If anything, for food. But one thing that TFP's will need to work on is keeping them from coming up and swimming on land.