Worth noting that the scene of throwing water on the witch doesn't play out just this way in the original book The Wonderful Wizard of Oz.
In the book, the Witch had held Dorothy captive and enslaved for some time, and via an iron bar made invisible by magic and placed where Dorothy would trip on it, she'd managed to get one of the Ruby Slippers off Dorothy's foot.
The water that was thrown on her wasn't a fire bucket, trying to put out the Scarecrow, but instead was the mop bucket Doroty was using to clean the Witch's filthy kitchen, and the explanation given in the book was that the Witch had been so wicked for so long that she'd dried up inside, like a walking pillar of salt, so that the thrown wash water just dissolved her (no "Melting, melting!" scene as she went).
Given that, in the books, everyone in Oz is immortal (no one can die in Oz, even if they're chopped up like Nick Chopper and Captain Fyter, with their amputated parts glued together to make Chopfyt), "given the gift of immortality" doesn't really apply in Oz -- but the Wicked Witches had suffered the side effects of their own wicked magic for a long, long time; hence why East just shriveled up leaving the Slippers empty after Dorothy's house landed on her, and why West melted when the bucket of mop water was thrown.