Fiberglass and tires float around too much. For creating a long-term reef, none of those materials is ideal. Reef Ball Development - researchers began realizing that the best reefs weren't made of junk tossed into the water, but objects that were specifically intended to become reefs. Barber chuckles at the thought. Plus, with concrete rubble, a barge is needed to haul it into the water, and officials have no control over the final height or shape of the pile.
Reef balls are far more portable.
With inflatable buoys, reef balls can be towed by a small boat. Deflate one of the buoys and the reef ball sinks to the bottom. Reef balls are good in shallow water, and concrete lasts longer than old tires or rusting ships. Old boats don't do that. Sunken ships don't do that.