Kaolin apparently has a specific gravity of 2.16–2.68, so it doesn't seem to be dense enough on its own unless the ratio of kaolin:water is extremely high? Good guess though!

My first guess was some form of alginate.

Alginate is probably not as dense as a suspension of kaolin, and also being an ionic polymer, there's no need for a surfactant or dispersant to stabilize it. Also it is probably way more expensive.

An organic compound would not have the density.