Check it out.
Dust of dryness. Absorbs 100 gallons of water, and condenses it into a small pill. Throw the pill and all water is released on impact. Use holy water. A direct hit by a flask of holy water deals 2d4 points of damage to an undead creature. Lets say a flask is one pint. If I'm not retarded, then you'll do, at maximum, 6400 damage to any undead creature you throw this pill at.
But it would also have consistent contact with enough being able to go into the undead being, even if the Holy Water evaporates on contact with an undead being it will be constantly hit with Holy Water from all sides. I also think that undead don't swim very well.
Yeah, that would be the best thing to do. I just like imagining a vent of Holy Steam constantly shooting outwards from an undead as s/he walks along the bottom of a man-made Holy Water lake.
This is an old as balls post, but there is rules of being submerged in harmfull liquids.
As long as you're submerged in a 1d4 acid, you actually take 10d4 (or was it d6? I can't remember) acid damage every turn untill you're nolonger in the liquid.
That might solve the problem. I'm not saying you're wrong or anything. =)
I just try to do what I can with what little I know. ^^