I put water in my MES 40 pan whenever I'm smoking something that I don't want to dry out. (Butts, Briskets, Chuckies, etc,etc...)
Personally I don't put the water in until after the first hour (I like to dry the outer surface a bit first), and I bring it to a low boil in the microwave before I put it in, so the smoker doesn't have to work harder heating the water.
Bear