Sand, water and the clay pot base serve one main purpose. They provide mass to the air column. When you have a mass that is heated to the set point temp of say 225* then, it act like a heat moderator. It helps suck up any big swings and slow any drops. More mass equals more stable. It's simple physics. So from that point of view it does not matter what the mass is. It can be sand, the clay pot, some bricks, rocks, or just about anything that will hold heat.
There is the argument that water in the pan not only adds mass to moderate the temps, but it also adds moisture to the chamber. The questions then becomes do we really need to add moisture? I've done butts with water, with dry pan, with clay pot, etc... and the all came out great. I will say this, you will use more fuel with water than dry or with the clay pot base. Reason for this is if you are cooking over 212* you are flashing water to steam and there is a big energy jump for that transition (ie, the water sucks up energy out of the air column when it flashes to steam in a boiling pan). This also contributes to water's ability to cool down a overly hot pit quicker than a non-reactive mass like a clay pot base. So for a new smoker owner, yes water pan with water is the way to learn the pit and how it responds. But once you have a good understanding of what to expect, you can go with other options and never look back.
I can't even recall when the last time I used water in my pan was.