kagis
Hmm.
It sounds like the water shortage thing isn't a recent issue on the island, but has been a long-standing problem stemming from a rapidly expanding population that's just been exacerbated by the cyclone:
https://www.cnn.com/2023/11/19/climate/mayotte-water-crisis-drought-climate/index.html
The island is grappling with its worst drought since 1997. Its two water reservoirs have reached a “critical level of decline” – one is at 7% of capacity and the other at 6%, according to the most recent estimates, and they are on the verge of drying up.
It has led to drastic water cuts. Residents only have access to water for around 18 hours at a time every couple of days, according to a schedule published by the Prefecture, the local subdivision of the French government. Many say what little water they have is often contaminated and undrinkable.
I don't think that shipping in bottled water is probably an economic long-term solution compared to doing desalination locally.
kagis
According to this, it looks like they're building a desalination plant. If it keeps to schedule, it's supposed to be operational in a year.
Stereau to supply Mayotte with 10,000 m3 of drinking water a day
The contract awarded to Stereau amounts to €36 million excluding tax. The project, which is expected to be completed by the end of 2025, will significantly improve the supply of drinking water on the island and reduce water shortages. The operation of the plant will be managed by Saur France for an initial period of three years, with the option to renew the contract for an additional two years, in one-year increments.
It sounds like a lot of the issue is that life in Mayotte, while maybe not fantastic compared to mainland France, is a hell of a lot more appealing than in a lot of nearby countries, so people from nearby poorer countries show up there, and the infrastructure hasn't been built out quickly enough to keep pace with population growth.
https://en.wikipedia.org/wiki/Mayotte
The issue of illegal immigration became very important in local political life in the 2010s and 2020s which led France to organize Operation Wuambushu.
In 2019, with an annual population growth of 3.8%, half the current population was less than 17 years old. In addition, 48% of the population were foreign nationals. Most of the immigrants come from neighboring Island state of Comoros, many illegally. Despite being France's poorest department, Mayotte is much richer than other neighboring East African countries and has developed French infrastructure and welfare system, making it a tempting destination for Comorans and other East Africans living in poverty in the region. The department faces enormous challenges.
According to an Institut national de la statistique et des etudes economiques (National Institute of Economic Statistics Studies of France - INSEE) report published in 2018, 84% of the population live under the poverty line according to French standards, compared to 16% in metropolitan France, 40% of dwellings are corrugated sheet metal shacks, 29% of households have no running water, and 34% of the inhabitants between the age of 15 and 64 do not have a job. These difficult living conditions mainly concern the large population of illegal migrants who crowd into shanty towns.