Here's what I reckon.
The 'item / Substance / material' you use, whether it be water, sand, bricks or clay pots, is for the purpose of drawing heat into the 'item' used, thus useing up some of the available energy (heat the coals generate) and keeping the general temperature in the BBQ lower than it would be if no 'item' was used?
The more 'mass' there is in the bowl, (water etc.) the longer the bbq it takes to reach it's optimum temperature (because it's heating up the water etc.) and also there is less fluctuation in temperature. (High / Low Spikes)
Water turns to steam at 100°c - 214°f so not good for crispy food if we cook at 225°f +
Sand would seem to me the best substance to use, as we can add or subtract the amount used, to raise or lower the temperature. (given that all other things are the same, time after time)
I think a close 2nd would be clay beads that you use for blind baking, they would be easier to clean.
just my thoughts