Shipboard potable water (drinking, bathing, whirlpools, etc.) either comes from a shoreside water treatment plant or is generated on board from seawater via Reverse Osmosis systems or Evaporators. Swimming pool water is typically seawater.

Cruise ships generate the massive amount of freshwater needed for showers, pools, and kitchens through two primary methods: Flash Evaporation and Reverse Osmosis (RO). Flash evaporation utilizes the "waste heat" from the ship's massive engines to boil seawater; the resulting steam is condensed into pure distilled water. The second method, Reverse Osmosis, is a high-pressure filtration process where seawater is forced through extremely fine semi-permeable membranes that trap salt and impurities, allowing only pure water molecules to pass through. Most modern ships in 2026 can produce over 500,000 gallons of freshwater per day using these onboard desalination plants. This water is then chemically treated and mineralized to ensure it is safe and pleasant for guest use. Additionally, ships can "bunker" water (fill up their tanks) while docked at a port, but this is usually more expensive than producing it at sea. To be environmentally conscious, ships also utilize complex "gray water" treatment systems to recycle water for technical uses, though the water in your shower is always fresh, newly desalinated ocean water.