Aqua Laguna

Aqua Laguna (アクア・ラグナ) is an annual high tide that takes place in Water 7. It engulfs the lower part of the city with a huge wave. Every year during the Aqua Laguna everyone is evacuated from the lower parts of Water 7. After each Aqua Laguna, part of Water 7 sinks. This causes the city to constantly receive repairing. Oda explained in an SBS that this came from the Acqua Alta (High Water in Italian), a similar phenomenon (but much less important) that happens in the Adriatic sea, and so in Venice, where it is an important problem due to the fact that the city itself is on the Venetian lagoon.

The water that makes up the Aqua Laguna comes from Long Ring Long Land in the once-in-a-year time when the sea level lowers exposing the ring of land connecting the islets among each other.

Since the Aqua Laguna is becoming more and more violent every year, the ground level of Water 7 is sinking day by day. To counter this, after the raid on Enies Lobby, mayor Iceburg decided to convert Water 7 into a ship, in order prevent the city from being destroyed.

Anime and Manga Differences
In an anime filler episode, it was shown that on the roof of the houses in the lower part there will be salt which is known as the salt from All Blue as the Aqua Laguna carries water from all the worlds seas, including the All Blue. This salt improves every dish, as such Sanji stockpiled it before leaving. However, this is non-canon.