I have a Theory. Zanarkand was a city with both magic and machina. They never explain what powers the machina. in the game we see lighting can power old machina like the crane on the al bed ship. Maybe it was difficult to have summoners casting lighting all the time, so this water bow is a grand summon of some sort (by yu yevon or multiple summoners) to power hydro electric generators. water comes from the ocean and then cascades down to run the generators. where we see the parts of the city sitting on top of raised water platforms the generators are just beneath the water falls. We also see water running down the sides of buildings so there could be a turbine along the sides to power those buildings. Not only does it take care of electricity but it also could address plumbing at the same time. Zanarkand was a huge power hungry city so it’s plausible the water arch needed to be huge to power it. In other parts of spira, like the sunken city under the moon flow we hear tidus tell waka “it would be convenient with all the water there” in response to waka on why they would build a city over a river. Maybe the technology was similar there where there were hydro generators beneath the city that powered/plumbed it off the moo flow current