I don’t believe anyone has a back up water supply.
La Terrasse and La Plage have two water sources. One for drinking and the other for flushing toilets. They store, treated recycled, gray water in tanks. That water is used to flush toilets. So that water would probably not disappear right away. However, those tanks would run out eventually if they were not able to be refilled with treated recycled water.
You would be left with bottled water from the market.
There has been no issue with the supply of drinking water any time that I can remember since Irma
With regard to electrical outages in the last week, the building generator has only kicked on once and that was last week late Wednesday night.
Drphilsxm