The problem with water not working is that it's not just the Wicked Witch who suffered from that weakness in the books. Singra was terrified of water, and Mombi was killed the same way. If I had to guess, I'd say the "truth" is that it has to be a very specific type of water. Like holy water or water taken from a pool in some far away locale. Water just not working at all would be really, really lame.