The water helps in adding some mass to the smoker in way of holding and maintaining heat. It also servers to keep moisture in the smoke chamber when doing this such as ribs, brisket, pork butts and so on. Depending on how large a smoker you have, you don't have to fill the area with water to the top. The area on mine is roughly 38 by 26 by 4" deep. I will fill it to withing 1/2" from the top. All the time the smoker is running at the temps I usually smoke at, 225-240, water is evaporating off so any juices and fats that drip in won't cause it to overflow. You will evaporate more water off that grease dripping coming in. (If not, you may want to trim some of the heavy fat off whatever it is you are smoking). Depending on the size again of your smoker, some people will simply fill one of those large rectangular throw away foil pans with water and set that directly under the meat they are smoking. Some will also use sand in those trays covered with foil. Once that heats up, it makes a great heat sink but provides no added moisture. Depending on how thick the plate is in your smoker will determine how quickly the unit comes back up to temps when you open the door, as will the thickness of the smoker walls.
