silicone cant cure underwater, so by the time the silicone drys the coral will too :/... IME the best glue is Zap gel. i dont know if its readily availible elsewhere but it is by far the easiest to use. epoxy will work but it ends up sticking like clay.. and if the frag is wet it wont stick on the way it should to prevent resession. try to get your hands on Zap gel, its convienient and actually cures faster underwater