I did find some information. Seems the use of a heat sink to stabilize temperatures is used in charcoal, propane and electric smokers. The use of water is great and assists in the smoke adhering to meat and helps maintain moisture. However, not necessary on large cuts with a lot of fat. Also, it stops absorbing heat at 212 degrees (elevation above sea level dependent) and evaporates. I suppose as you lose water to evaporation, you lose mass and your sink shrinks.
Dry playground sand is reportedly much better. It will continue to absorb heat, all the heat you give it. And it does not evaporate. It can be an aid in crisping since it is a dry heat. And if additional moisture is needed, just add water to the sand. If dry, you can cover the pan of sand with foil to keep grease out of it.
Bottom line: each method has its own adherents. And they like the results they get. And they politely disagree as to which method is better.
There are also reports of employing lava rock, ceramic tile, and other means of absorbing heat and releasing slowly.