You could use the greatstuff expanding foam, but that will add bouyancy issues. If you do use concrete/cement, you will probably have to seal whatever part is exposed to prevent it from affecting pH and hardness, in addition to waiting for it to cure.
A while back, I had seen an epoxy grout mix in Lowes or something, but I never got around to researching it, I've been wondering if it would be useful for creating backdrops/artificial rocks.
I'd strongly encourage you to make your own - get some epoxy, some sand, and some foam, it's not too difficult. Just stack/carve the foam to the rough shape you like, and then coat it with an epoxy/sand mix.