Most are tied to living coral. This obviously damages the coral and in many cases breaks large pieces of coral completely off of the reef.
In the past, moorings were tied to concrete blocks which turned out to not be heavy enough for the heavier boats when the seas were up, thus were dragged around the bottom causing even more damage to the reef. This also creates a hazard for the boats which use them. So history has shown that tying moorings to coral or concrete blocks is not the answer.
It would seem that we need not only more moorings, but moorings installed in a way that does not cause more damage to an already ailing reef system.