This already exists: dust of dryness, DMG p. 166. (SPOILER ALERT: wondrous item, uncommon)
EDIT: It's not exactly the same, because you have to smash the pellet to release the water, rather than reconstituting it with water. But that means you can use it in cool traps and stuff. One time my brother's character got a hold of some dust of dryness and made pellets, which he then affixed to the end of crossbow bolts. Several months later (real-time), after everybody had forgotten about this, he used the bolts to absolutely annihilate some fire elementals, in what would have otherwise been a very difficult encounter.
