In my experience, doing hundreds and hundreds of pounds of pulled park with water vs with sand in the WSM, I don't see any advantage to the water in the finished result. There's also lots of videos, message boards, and blogs that have done side-by-side tests. It takes more energy to heat that water up which means more fuel burned. Maybe you get some moist air on the rub for bark production, but even that's a stretch.
Do some cooks one way then some the other and see your results. In the end, the beauty of Q is that you get to mess with the variables and make it your own.