The Artificial Reef Program used four types of obsolete Army armored vehicles as artificial reef materials off the New Jersey coast. These were cleaned at local military bases, loaded onto barges for transport, and pushed off at their final destination. Once the Army had disposed of its excess inventory, the program ceased, around 1999. The Artificial Reef Program has sunk almost 400 tanks altogether, far too many to list them here in this website.
There is a great deal of controversy over the use of subway cars as artificial reefs. There shouldn't be. Subway cars are fish condos. They are the perfect size and shape to provide homes for all sorts of fishes, as well as large attachment areas for other organisms. The fact that they come complete with large door and window openings is even better. Most reef materials, such as ships and barges, improve with age because they open up, allowing easier access to the interior. Indeed, some of the most barren reefs I have seen are those that are completely intact, since they offer little shelter.
All manner of concrete, steel, and stone rubble from dredging, demolition projects, and other construction is used as artificial reef materials. This material is generally available at very low cost or free from construction companies who are more than happy to get rid of it. Transportation costs determine where this material is used by the Reef Program.
Scuba diving in the United States is by and large not regulated by the government ( as opposed to Australia and other places, where it is. ) This remarkable fact is the result of the scuba industry's so-far successful efforts to head off government meddling by putting forth their own standards for training, equipment, and other specifics. In fact, all the major diving certification organizations and manufacturers actually got together and agreed on a basic set of requirements for the industry, and even have an ISO-9000 certification for it.