On the Southern California coast, they've dismantled some old oil rigs too. But they were allowed to leave a certain amount of the infrastructure intact, about 70 feet or so below the surface, because there was so much marine life clinging to them. They became artificial reefs.
In fact, there's enough interesting sea life there to support recreational diving trips to the sites.
https://www.nytimes.com/2016/03/08/science/marine-life-thriv...
http://www.scubadiving.com/travel/pacific-western/southern-c...