Most ships can store 500,000 gallons of fresh water in massive tanks located in the hold area. After guests have used the water for showering, toilet water, laundry, etc., the crew treats it before releasing it into the ocean. A cruise ship will bring fresh water onboard when they visit ports.

A large, modern cruise ship like an Oasis-class vessel can hold approximately 500,000 to 1,000,000 gallons of fresh water in its storage tanks at any given time. However, the total amount of water "processed" is much higher. On average, a cruise ship passenger uses about 200 to 300 liters (50–80 gallons) of water per day for showering, laundry, and dining. To meet this massive demand, ships do not simply "carry" all their water from port; they are floating water treatment plants. They use "flash evaporators" that utilize waste heat from the engines to boil seawater or "reverse osmosis" systems to filter salt and impurities at a high rate. A single ship can produce over 500,000 gallons of fresh water per day while at sea. This allows the ship to keep its tanks topped up for pools, spas, and kitchens. Furthermore, the ship must manage its "ballast water"—millions of gallons stored in the hull to maintain stability and balance—which is separate from the potable fresh water used by the thousands of guests and crew members on board.