"Thinking about using it to attach suction cups to powerheads unless someone has a better idea?"
I would attach them to mag floats instead of suction cups. They cost a few dollars, but it will insure that your PH never comes loose beause the suction cups fail.
Another tip for gluing underwater is to sort of massage it on the place where your adhereing it too. This should allow more of the glue to work its way in to the rock pores and anchor itself. Once there appears to be enough glue to rock contact I hold it in place for about 2 mins. The hardest part is fending off the cleaner shrimp so he doesnt get his claws full of glue.