Butter-water will just be a mess, and the butter flavor will be lost.
Our suggestion - which doesn't really give you an answer you want to hear - is to invest in a few of the specialized corn butterers on the market: Butter Boy, Butter Spreader, or Corn Butterer, that people can use as they pass through the buffet line. Or you can buy individual pats or packets of butter from a warehouse club and keep them on ice near the corn station. That way, at least, your guests can make their individual messes at the table rather than at your buffet station.
The other choice is to follow Shirley Corriher's lead and mix up a quantity of melted butter, salt, and pepper (two teaspoons of salt and one teaspoon of pepper to a pound of butter) in a separate dish and let people roll their corn in that before they place it on their plates (or you can use tongs to roll it for them and place it on their plates, again, managing somewhat that mess thing).