I heard one idea of putting a “skirt” around old oil rigs, the purpose of which would be to catch and deposit an increasing amount of inorganic material, slowly building to a steep peak, with the rig atop it. By that time, the rig would have decayed. But if designed well, the peak would continue to widen, creating an underwater island conducive to life.
Other things could be added, like scuttled ships, so that eventually a reef would start to form as well.
That would take FOREVER, even in 100ft water.