I have a water smoker, but haven't used if in a few years, though I always had fairly good luck.
In a water smoker, the fuel is located in the same area that the food you are smoking. Though you can keep a lot of the heat down by controlling the damper, it's hard to get it down low enough to slow cook, or below the temperature of 225 degrees. Therefore a water pan is added, right above the fuel source; and the added moisture helps keep the temperature down. Another added feature, is that you can add wine, marinade, herbs or what every is your choice to help inhance the flavor of what you are smoking. It takes awhile to get use to using, and you need to be careful if more water has to be added, and depending on the design, it may be difficult to add more fuel.
