I'm in a bit of a hurry this morning, so if this post comes off as abrupt, I apologize.
There are only two confirmed reasons to use water in your water pan. First, to maintain low temps. Water is a heat sink. Two gallons of water is a 16.7 lb mass that has specific properties of heat absorption and expansion. Personally, I've found that dry smoking is a MUCH better teaching tool for learning to control chamber temps than depending on the water, water that will eventually evaporate and cause spikes in chamber temps. Using water also uses more fuel because you have to use a lot of your heat energy to heat the water.
Second, a moist environment causes more smoke to adhere to the meat, which can be both good and bad. If you have "bad" smoke (white or grey and filled with particulates), you get an ashtray taste on the meat, or a bitter creosote flavor. If using a strong wood like hickory or mesquite, and the smoke is good (thin blue), the smoke flavor can overpower the meat. If you want a lot of smoke flavor, and moist meat, spray it instead.
Some believe a moist environment keeps the meat moist. True and false. True in that the outside of the meat can be kept softer if using a water based spray that does not contain sugar or any other ingredient that will burn as it evaporates. False in that spraying the outside of the meat does nothing to keep the interior moist, and there are posts I've read where folks believe it can actually contribute to the interior of the meat being dryer due to the evaporative flow it creates as water surface moisture leaves the meat. Since a lot of the meats we long smoke do not get their juiciness from water, but from melted collagen, the evaporative flow probably only applies to lower temp meats like roast beef, tri tip, etc. Personally, I only spray if I want to add a specific flavor addition to my rub's flavor profile. A double smoked ham is all I spray these days, but that could change.
