The answer is that cruise ships always have access to fresh water that's obtained through several different techniques. For instance, some of the more modern and sizable cruise ships produce their own fresh water onboard via a desalination plant.

Yes, modern cruise ships are highly self-sufficient and produce the vast majority of their fresh water while at sea using two primary methods: Reverse Osmosis (RO) and Distillation. Reverse Osmosis is the most common and energy-efficient method; it involves forcing seawater through microscopic, semi-permeable membranes that trap salt and impurities, leaving behind clean, fresh water. Distillation (often called "flash evaporation") involves heating seawater to create steam, which is then condensed back into liquid form, leaving the salt behind. Once produced, the water is mineralized and pH-balanced to ensure it tastes like standard tap water and is chlorinated for safety. These systems are so effective that the water in your cabin is often cleaner than municipal water on land. Ships also have the capacity to "bunker" (pump in) water from port cities as a backup, but they generally prefer their own high-quality, on-board production.