IMO if you want something to actually stick to the rock epoxy is the way to go... but it has to be aquarium safe...
HoldFast Epoxy Stick
Aquarium Aquascaping & Coral Propagation: Two Little Fishies AquaStik Epoxy Putty
I have used both and they both work equally well... and they cure underwater
Super glue gel will not hold for very long especially on rock but you can use it underwater