I've had good luck with Instant Ocean Hold Fast, similar to AquaStik. It's a two part epoxy putty that bonds underwater. As long as you combine it with a cyanoacrylic super glue sandwich, it bonds well and dries quite hard.
I haven't found an epoxy putty that bonds well on it's own. I've only found it effective using a layer of epoxy putty with cyano-acrylic super glue on the surface bonding to the rockwork and on the surface bonding to the coral rubble or whatever is being mounted. The epoxy provides a modable surface to match the rock work, but the bonding is better left to superglue. I had a lot of early, failed rock-to-rock bonding when I did my initial aqua scaping using epoxy putty alone. Better research would have prevented that.
I bought a bunch of the white colored epoxy, but wish I'd gone with the coraline purple. The white stands out against living rockwork until things grow over it. A purple or a greyer color would blend better.