I am kind of on the fence with Fat Man & AdidaKev, too. I definitely see your heart is in the right place, but let's say there's some unknown, like some rare goby that likes to inhabit flat open ocean floor, and you are piling cement on that unknowingly. Honestly, in reality I think the chances of that are very unlikely (especially since you are working on such a small scale), and you are probably making a barren area into a mini-reef that can support a richer microcosm; it's just more of an unknown when it's done on a private, hobbyist scale, a little different to me than when the govt sponsors sinking a ship to create an artificial reef in a well-studied area.
I guess with that one reservation, it's probably on such a small scale, in practice I could see more good than harm coming from such a project. I know that private mini reefs have been created for years off the coast of Florida & Alabama by fisherman sinking all sorts of objects to try and promote fish populations coming closer to shore; but to me, you have nicer aspirations, and what you're doing is thought through better (sinking cement, rather than old cars and random materials, which was what most of them were doing). Let's see some photos if you ever get them.
