There already exists a method for creating artificial reefs by using electricity and this is patented however I believe there is at least one other way of doing so that may be more efficient. Please consider the following:
Coral reef is mostly composed of calcium carbonate. The formation of calcium carbonate occurs in different ways. One way is time, another is electricity and last but not least heat.
Since the electricity method is patented I believe the next plausible method is to use heat.
Another interesting thing to note is the fact that while calcium carbonate particles in saltwater are in the presence of existing calcium carbonate crystal formations, an increased accumulative effect is observed.
I believe that a canvas like material that is intertwined with steel wire or chicken wire or thin steel fencing may serve the purpose of being medium for growth of calcium carbonate. Galvanized may work better since zinc is a very active material so perhaps that is more suited. Also since already having calcium carbonate in saltwater promotes growth of more calcium carbonate I would say that the canvas and steel material should already have a thin layer applied before submersion.
I think that two long sheets of this material that are parallel to each other (maybe a foot or less apart) with a steam pipe on the bottom between them may be a plausible method for promoting the generation of artificial reef.
If the canvas type material doesn’t work than maybe something like a steel or galvanized mesh will. And not to mention you could always layer it.
As far as heat transfer of steam and loss of energy I think this can be a problem. Steam would have to be superheated and the carrier pipe would likely have to be made of or encapsulated in a material that retains heat.
As the heat rises the fabric and/or metal mesh absorbs in the upward path and the calcium carbonate will deposit on it. A wider opening at the bottom of parallel sheets or mesh and tapering near the top. So the wall would have a wider base at the bottom and taper to lessen width at the top. Holes in the bottom of the fabric would be necessary to include nearby saltwater to harvest calcium carbonate from.