With most cooks, I do not use a water pan. I have a 885 and do most of my cooks on the top rack. I place a foil lined baking sheet on the bottom rack to catch the drippings from the cook. That way, I can roll up the foil and toss it in the garbage. I do not have to clean the Traeger drip tray, grease chute, or grease bucket. That make the clean-up job a breeze.
For longer cooks, I do occasionally use a meatloaf pan half full of water. If I do not use it, I will occasionally spritz the meat with a combination of apple cider vinegar, apple juice and water in equal portions.
Those living in an arid environment like Phoenix, Denver, Salt Lake, San Diego, or West Texas, the water pan might be needed, but in Richmond, VA, it is less necessary.