1:125 scale.....sunken wreck.....OK there would be some coral, as well as barnacles, urchins, and other assorted sea life encrustations, and the ever present silt. So far, some good recommendations. What I would add is, when applying all this stuff, keep it relatively small (the Great Barrier Reef wasn't created in a day). Apply glue, and then sift the material over the model. When the glue is dry, dust, or vacuum the excess off, and do the next application. And so on. The final touch will be the silt. this should be a fine dust, and I would overspray everything with a light coat of Testors glosscoat, and blow the silt material above the model, letting it settle down from a foot or two above, layering until the desired effect is achieved. When everything has been applied, and is done drying/curing, apply a final coat of testors dullcoat. The whole idea of "sifting, and settling" is, that how it happens, and the whole display area should have the same overall look and feel, especially with the silt.