I use water on the 18.5 WSM, I foil the outside with two layers, and I only fill the water pan about 1/4 full for Ribs. 1/3 to 1/2 full for butts and briskets. To avoid the mess I spread out a sheet of foil on the lower grate and bend the edges up about 1/4". This catches all the mess and I have found that the water pretty much evaporates at the end of the smoke so I don't worry about re-filling. If I need to use both grates I use a rack on the bottom on top of the foil same result. YMMV.