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.
The "Ocean Prince" drydock (above) was sunk in 1986 and is well-attested. New York has photos of a clearly different drydock which they place here, but I believe is actually at Shinnecock, and I have placed it there.