Any Cyanoacrylate glue should work.
http://en.wikipedia.org/wiki/Cyanoacrylate
From the Wikipedia article:
"Cyanoacrylate glue's ability to resist water has made it popular with marine aquarium hobbyists for fragging corals. The cut branches of hard corals such as Acropora can be glued to a piece of live rock (harvested reef coral) or Milliput (epoxy putty) to allow the new frag to grow out. It is actually safe to use directly in the tank, unlike silicone, which must be cured to be safe. However, as a class of adhesives, traditional cyanoacrylates are classified as having "weak" resistance to both moisture and heat[5] although the inclusion of phthalic anhydride reportedly counteracts both of these characteristics.[6]"