In the Shustermans' Dry, a water-zombie is the name given to someone who's gone without water for so long that it's starting to affect them mentally and physically. As the water supply dries up, more and more people become water-zombies, desperately wandering across the parched landscape of California.
If you've ever seen a zombie movie, you'll know that this is how zombies normally behave. The water-zombies in Dry aren't actually zombies, of course; they haven't come back from the dead. But they certainly give the appearance of zombies, so damaged as they have been by a prolonged lack of water.
Water-zombies represent a real threat to everyone else in society because they are prepared to do literally anything to get their hands on water. Whether it's trampling an old man to death, stealing water from a child, or evening stabbing someone to death in cold blood, there are no depths to which water-zombies are not prepared to plumb to satisfy their thirst.
As well as trying to avoid water-zombies, Alyssa, Garrett, and the others have to face the danger of becoming just like them. As Jacqui says, if you're another thirsty soul in a sea of water-zombies, you become one. It's all too easy for someone to go out of their minds due to lack of water. Before long, they can find themselves just another member of the rampaging mob of crazed, thirsty, desperate people who've lost their basic humanity.