Aqua Laguna

Aqua Laguna (アクア・ラグナ) is an annual high tide that happens in Water 7. It engulfs the lower part of the city with a very big wave. Every year during the Aqua Laguna everyone needs to move to a very high place. After each Aqua Laguna, part of Water 7 sinks. This causes the city to constantly have to be repaired. Oda explained in an SBS that this came form the Acqua Alta (High Waters 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 venitian 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. So, it is possible - although never stated - that last year's Aqua Laguna was bigger than usual because of the large extent of sea that was frozen by Aokiji some days before.

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 considered non-canon.