On the surface, that would be a great idea--but there's the problem: All that water, when it's flooding, is indeed ON THE SURFACE. Which means that we'd need enormous "water catchers" to contain the stuff before it could get away.
Oil is already contained (in rock formations far below the surface, usually) and therefore easier to KEEP contained, since it's never been known to "flood" before people were ready to put it to use.
Oil wouldn't be easy to handle, either, if it started out by falling from the sky and then just ran downhill.
Then again, I'm willing to bet we'd already be moving water all over the world if it could be sold for the current oil price of around $102 per 42-gallon barrel.