rock work and structers

I am currently in the planning stage of my next tank, and this time i want to spend more time planning my rock work.
You see in the news where they will sink old trains planes and ships to use as false or man made reefs, and i have been pondering how one could go about recreating this, as i would love to have a old tall ship like structure in my build.  Things i have considered are, take blocks of foam and carving out what i want then covering it with morter and crushed coral. After further thought i dont think this will work simply because it wont be pouris enough. I have also thought about using base rock and a dremil to do this making several sections of it and placing them accordinly.
do you think this would work, or have any ideas that would work or thoughts on why something like this would be a bad idea? I would hate to put this together to find out theres a reason you dont see or hear more about this type of thing.

 

yeah thats why i opted against using the foam and such.

 

thats not a bad idea bela, like make sort of a stage front