The floats are encapsulated polystyrene foam. Made specifically for floating docks. They are almost indestructible, and if they ever do get a hole poked in the shell it doesn’t hurt the bouyancy.
I thought about buying some plastic drums and foaming them, but I found exactly the floats I needed, at exactly the time I needed them, on the facebook marketplace. Guy tore out an old dock and wanted to get rid of the floats. I bought enough to float my entire dock for less than one brand new one would have cost.
Love to hear about deals like that.