Instead of R'lyeh, maybe Devil's Reef is more do-able? Now this is just some random musings after re-reading The Shadow over Innsmouth, mind you....
First thoughts: We know that the reef is a mile from shore. OK, fine. Perspective wise it seems to be visible, but not too big. Now something six feet tall looks about a quarter of an inch tall for argument's sake at a mile. My scaling is probably wrong, but lets go with that for now. So the reef probably is about 6-12 feet out of the water. Well 2" blue foam would serve fairly well for our purpose it would seem.
Second Thoughts: What exactly does a rocky reef look like? This isn't coral, so not jagged. Smooth perhaps, or even rough like sandstone. Possibly with some pockets of harder or softer material that hasn't eroded, or has left strange depressions. Hmmm....... Irregular lumps of rock with pockmarks then? possibly some strange altar, with low rocky outcroppings surrounding it?
Just some musings.