The Aegean islands are partly supplied by local groundwater sources. However, some smaller islands have insufficient water resources and have to be supplied via tanker ships at a high cost.

Because most Greek islands lack large natural lakes or rivers, they rely on a combination of desalination, tanker imports, and groundwater. In 2026, desalination has become the primary solution for popular islands like Mykonos and Santorini. These plants use "Reverse Osmosis" to turn seawater into fresh drinking water, though this process is energy-intensive and often supplemented by solar or wind power. Smaller or more remote islands still rely on "water tankers"—massive ships that transport freshwater from the mainland or larger islands. Many traditional homes also use cisterns to collect rainwater during the winter months. However, the combination of tourism growth and climate change has put immense pressure on these systems, leading to a "water crisis" in some regions where groundwater is becoming "salinized" (salty) due to over-pumping. Most islands now strictly advise tourists to avoid wasting water and often provide bottled water for drinking.