It seems since the WSM was introduced in the 80's there's been a water vs sand vs pebbles vs bricks vs terra cotta clay flower pot saucer in the water pan debate ongoing here and other web sites. Who's "right"? I've used water with success. I've used nothing with success. I've used a terra cotta clay flower pot saucer with success. What's "right"? My take on the subject: different smokes for different folks. Whatever works for you. Your mileage may vary.
The mythbuster of the BBQ world adds to the debate with a pretty good read on the subject. In part the Meathead (from Amazing Ribs) article says:
Water pans. Water pans are not for making gravy. In my articles on the best setups for different grills and smokers, I advocate using a water pan under the meat or over the fire. Here's why:
1) Water in the pan absorbs heat and never rises above 212°F. This helps you keep the temp down to 225°F, a temp I recommend you learn to hit with regularity (read my article on calibration).
2) Water helps stabilize the temp in the cooker and minimize fluctuations because water temp takes longer to rise and fall than air.
3) A water pan can block direct flame when you need to cook with indirect heat.
4) Water vapor mixes with combustion gasses to improve the flavor.
5) Water vapor condenses on the meat and makes it "sticky" allowing more smoke to adhere. This smoke enhances flavor and sodium nitrite in the smoke creates the smoke ring.
6) The pan can add humidity to the atmosphere in the cooker to help keep the oven from drying out your food. This can vary significantly depending on the design of the cooker. If you place a water pan a few inches below the meat in a pellet cooker, the water does not get very hot, and it will have little effect. In an offset or bullet smoker, with the water pan directly above the coals, it can make a significant difference in how much water evaporates from within the meat and how moist the meat will be.
7) The humidity keeps the meat moister and that slows cooking as the moist surface evaporates and cools the meat. This allows more time for connective tissues and fats to melt. Humidity can also help with the development of the smoke ring.
Try to use hot water. Cold water will cool your oven down a lot and should only be used if you are running hot and need to cool it down. And fill the pan to just below the lid so you don't have to keep opening the lid to refill. Put it above the hottest place in your cooker so more water will evaporate.
What goes in the water pan?
Pitmasters argue over what should go in the water pan. Not surprising since we argue about everything, even the meaning of the word barbecue. Some say beer, wine, apple juice, onions, spices, and herbs. Some folks like to put sand, dirt, gravel, or terra cotta in the water pan. What works best? There's a reason it is called a water pan.
No sand, gravel, etc.
Many weekend warriors like to put sand or gravel in their water pans. Solids do nothing for the humidity, or for the flavor. They may help stabilize temperature fluctuations, but they will not keep the temp down like water. Water will not go over 212°F. Sand and other solids will heat up to whatever the oven temp is. So if you have a charcoal bullet smoker or a gas smoker that tends to run hot, say 250°F, but you want to be at 225°F, then water will help you keep it down. Eventually the sand will warm to 250°F.
Drink the beer
Drink the beer. Drink the wine. Drink the juice. Put the spices on the meat. Just use hot water. Don't waste your money. Many of the compounds in these other liquids will not evaporate and even if they do, they just make no impact on flavor. You may be able to smell them, but the number of flavor molecules in beer, wine, or juice are so few that even if they were deposited on the surface, they would be spread out so thin you would never notice them. The flavors of the spice rub you put on the surface of the meat, the smoke, and the sauce you chose, are much much stronger and will mask any molecules of apple juice or whatever else is in the pan that might alight on the meat.
The AmazingRibs.com science advisor Dr. Greg Blonder says "If you want to increase humidity, and you do, fill the pan with those red lava rocks sold at garden stores, and then add the water, but don't cover the rocks. They are very porous so they act like sponges, and the large surface area pumps more moisture into the air. And don't let fat drip into the pan because it will quickly coat the surface and prevent evaporation."