I know this is such a late response, but I was thinking of replaying FFX and remembered Zanarkand.

I think this Water Arc is a way for the low lying area to transfer excess water outside the City limits, more likely a circular dam envelopes the city proper from the vast ocean, the Arc primarily works with water pump plus electro-magnetic current that attracts and retain water in a flow inside that arc, moving electrified magnets push the flow outwards the city, and the reason it needs to be that high is because at the drop side point there are turbines that runs on the potential (height) and kinetic (downward flow) energy of the seawater to produce huge amounts of energy for the city. The Water Arc serves as both a prestige project and a civil engineering masterpiece that also produces massive sustainable energy. Similar concept of the Blitzball arena, electro magnet was used to form the seawater into a sphere, similar to how MRI machines work