The water pan is a heat sink. What material it is does not matter as long as it holds the heat consistently. Water, ironically, in a water pan does not hold the heat consistently. As the water evaporates from the heat, it becomes less efficient.
Enter other materials. Saiko uses sand. I wrap the pan as he does. Instead of then filling with sand, I place a clay flower pot base in the pan. It too is wrapped in hd foil. I then wrap the pan and the flower pot base in another layer of foil. That is the one sheet that is removed after the cook and discarded.
It provides the same effect as sand, without the possible mess of sand getting on food.
I have never used water in my WSM.
B4 clicking reply, I see I just used more words to say the same thing Rich Parker did. Oh well . . .