I've used tubes of this stuff to help hold rocks together and to mount corals as well. I don't know if it's really salt water safe or not, but I've been using it for 10+ years without any noticable ill effects. For mounting frags, I'll put a little superglue on the frag plug, then some of the epoxy, then some superglue on the epoxy. That hole mess gets shoved onto the rock I want to coral mounted to and it sticks right away. I'd think it'd work well for your purposes.
Good luck.