Gorilla Glue is amazing stuff. It's really made for waterproof bonds on wood, but it's supposed to work on most anything. And water does activate it.

However, there's also Pool Epoxy, which is a putty that can harden underwater as well. I would probably have used that first, after drying the unit out. Were it an automatic erosion chlorinator I would do that.

But with Nature2? I would DEFINITELY take the sledge-hammer approach!