Getting rid of old ships is very expensive, even if they are sunk as a reef, as they are usually filled with environmental elements that have to be removed and disposed of individually with personnel and equipment that is certified for the effort. Items that come to mind include, but are not limited to asbestos, lead paint and PCB electrical items. If the vessel is to be sunk, it also must be clean of all lubricates (tanks, machinery and bilges) as well.
Five years ago, while working on a project to install a new crane on one of my vessels at work, I found a (very) small area on the crane deck that had lead paint in it...it was a show stopper that cost upwards of $30K to dispose of.
In the Coast Guard, I was involved in removing asbestos and lead paint from ships in the late 70's and the cost even back then was breathtaking.