Reanimate is gross too. But necros being able to
easily raise zombies whose level is much, much higher than their own was probably never part of the design of the class.
As long as NPC undead are invulnerable (or really, really resistant) to certain damage types there will be the potential for abuse. Remove the water invulnerability and the problem largely goes away.
Or, as a bandaid, just make it impossible to raise a zombie from any NPC that lacks a metabolism. But that leaves open the possibility that some necro will discover some OTHER mob with a watter attack that can be exploited.