I certainly keep my water pan filled through out the whole smoke... i tend to use it as a crutch though to maintain temps in my offset... this could be very much in my head but i think (without having a roaring fire) that it acts as more than just thermal mass and really prevents it from going above about 250. maybe it just works that well as a thermal mass, but something like sand or gravel doesn't seem like it would act as well as a heat sink since the water is constantly absorbing tons of heat in it's liquid to gas switch and being frequently replenished compared to say sand or gravel which would take additional heat for sure but would more so just slow temp changes. this also allows me to have a cleaner burning fire and not have to pre-burn wood or use charcoal and have little to no air intake restriction.
If you ditch the water and trim back your fire you'll use FAR less fuel. And, your meats won't dry out. I did the same thing back in the day with my SnP 40"...that hog would boil-off a gallon of water in 2 hours and I had condensed water dripping out the bottom constantly. In my Brinkmann Gourmet I learned to ditch the water and I'm doing the same thing with my WSM-18. Yes, water will absorb heat...that's the start of the problem, though. It absorbs thermal energy only to waste it as evaporated water, which in turn, cools the smoke chamber...wasting fuel. Water is not a thermal mass or heat sink...it's a high-temp spike buffer...that's all it's really good for.
Sand or gravel is a true heat sink as are fire bricks, lava rocks and any other mass you add. If you want more stable temps while reducing fuel use, mass is the answer.
it does make a lot of sense if i was more intensively controlling the temps through the intake to slow the burn of charcoal or by turning down a gas burner, but restricting a raw wood fire seems to produce a bit of creosote in my experience... on the other side of this, i don't see places like Franklins using water pans... maybe the sheer size of their smokers allow for a nice clean burning wood fire without as much risk of exceeding the desired temp range, or maybe i'm just lazy and haven't master how to control a wood fire with air intake without producing heavier smoke than i desire. I do appreciate the insight and will certainly experiment with other ways of maintaining temperature without the water pan crutch.
If you're burning raw wood, then you need a smaller, hotter fire to reduce production of soot. Use smaller splits and add fuel more often as dictated by your chamber temps, but a hot fire burns cleaner. Big smokers need a bigger fire, but they still burn a hot fire, and all the mass of their cooker helps to maintain a stable temp. If you have a 2000lb cooker beside a 200lb cooker, you know which one will have less temp peaks and valleys.