Here's a picture of how potable water gets to Filicudi today.
It would be much less expensive to build a desalinator... but perhaps politics think that paying someone to refournish the island with water by carrying it from Palermo or Messina is a better idea... uhm... yes... it is still good old Sicily !!
Anyway.. in Filicudi water arrives by those ferries, then is pumped on a big tank and then distributed to various regions of the island for a few hours every week.
Everybody collects rain waters in wells, so the need for water is mitigated.