A thought just occured to me the other day as I was reading back through the Water 7 arc. Perhaps Water 7's huge fountain gets it's water from Enies Lobby's giant wanterfall. All that water has to go somewhere, right? And it seems the falling water has enough pressure to travel through a series of caverns underneath the ocean floor that open up on Water 7's Island, thus creating a fountain.
It's highly unlikely that the technology exists to create a pump large enough to run Water 7's fountain, so that's out of the question.
What are your thoughts?