We have a couple of threads that cover that. Somewhere in the archives. We have basically determined that anything that goes in there does not impart flavor to the meat you are cooking, but gives a nice odor to the surrounding area and will make the neighbors come out in droves. The purpose of the waterpan is to both act as a heat sink (helps stabalize against temperature swings), and to add humdity to the smoke chamber.
Some of us have switched over to sand in the pan. This allows the heat sink capabilities, and when covered with foil, minimizes the clean up afterwords.
Find those threads that discuss that and then make your own mind up.