The water pan does two things for me, since my smoker is a huge upright I built myself and I use charcoal/wood. It acts at a heat shield to keep the direct flame from cooking the meats too fast. Recently I've started putting down foil under the water pan, which is a turkey roasting pan, to cover the rest of the grate that it sits on, with the only opening the outer two inches of the grate. Far easier to control this way, especially with something like albacore tuna that only smokes for a couple hours on a very low heat. The pan also provides steam to keep things moist, and help the smoke penetrate the meat. I still get a little more heat on the bottom racks of course, but with the foil it is a lot more even top to bottom than it was before.