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.
How do two ships in the wide ocean collide? It seems unlikely, and yet it happens all the time. Often, the ocean is not all that wide. Many collisions occur in shipping lanes and port approaches, where ships are brought together in close proximity. Here are some videos of actual collisions between ships: