A lot depends on where, how and for what reasons it was done. Artificial reefs,
FADS ( fish attracting devices), have been proven to aid in the production of marine life. When AC had 700 people living off of the sea, the natural marine ecosystem provided them with all the sea food needed. Now with 20,000 people putting that kind of demand on the system, destruction of the system due to development and part of the system, i.e. reef, off limits to fishing, there is an argument for man having to supplement nature.
One of my more productive fishing sites on the back side of the island is an old truck bed that was placed there years ago. The area on the back side of the island lacks in natural structure providing protection and production of marine life based on the amount of fishing pressure that it has now.
Dumping garbage just for the sake of getting rid of it, or dumping it in the middle of Hol Chan would be wrong. But before jumping to the conclusion that it is wrong, one should find out more information.