Even on a cool day like today-- around 60*-- one full Weber chimney of charcoal will bring the temp of an empty WSM to 325* with a dry water pan. Filling the water pan gives the equivalent of about 8 lbs. of meat's worth of heatsink, which will allow the cooker to run at around 250*, vents wide open, given the aforementioned ambient outside temperature.
Using sand will give the same result up to a point, but sand can continue to absorb heat and rise above the 212* that water is limited to, reason being that, at 212*, water turns to vapor, and, therefore cannot rise any further in temperature.
So it's not "cheating" to use water, or any other material for that matter, in the pan. On the contrary, the WSM-- and all "water smokers"-- are designed to use some form of heatsink to maintain temps in the "low & slow" BBQ range.
I wouldn't count on water vapor maintaining, or contributing in any significant way, to the moisture level of the meat-- it's the fat content that performs that function in true BBQing.