I found out today that Moon Sand is made out of sand that is coated in a chemical compound which makes it hydrophobic , or water repellent. 

History

Moon Sand was also known as Magic Sand, which was originally made to help clean up oil spills, in the ocean.  It was used for oil spills, because once it has contact with water it starts to stick together.  Thus creating a layer of sand that will sooner or later get to heavy for the water to hold up, so it sinks with the oil as well.  Moon Sand or Magic Sand was actually created around 1970's to 1980's.

Adding to my research, I will try to figure out the maximum weight it can be before it sinks to the bottom of the water.