I only discourage those characters because a huge chunk of my setting is desert and the other regions essentially surround that desert and at some point players usually find themselves crossing it - if only for a short time - which puts them in a position of having to parse water between drink AND douse. Not that that is such a huge thing to overcome, but the desert plays a big enough role in the setting that it could become a hassle.
Discourage doesn't mean ban. It's just giving the player all the facts.
Likewise the bestial (centaur, minotaur, etc) characters. There is a whole wide gameworld to play them in, and I've run plenty of campaigns with bestial PCs, but the NPC reactions in cities and towns can become prohibitive. Again, it's just laying everything out for the player up front before he makes his decision.
If a PC escapes from the Cleaves with the aquatic mutation, they will move to the coast and stay there. They could also seek a caster to polymorph them back into a normal human.