Can’t really comment on anything above the waterline but below typically they become more attractive to marine life, basically things grow on them... if this is good or bad depends on who you talk to (eg should barren places remain barren, is increasing the amount of marine life the right thing?).

I worked on a project decommissioning an oil rig d as no it nearly got canned because rare cold water coral started growing was found on the structure. Marine life is weird...