How the Ocean Becomes Fresh Water.
We were very honored recently to receive a special guided tour of the Engineering Control Room by the chief engineer on a major cruise line’s ocean-going vessel. We were shown the operation of a state-of-the-art water treatment plant aboard ship. (See photo: “The Davies in the Engineering Control Room of a modern ocean-going cruise ship.”) A modern cruise ship is essentially a floating city. Thousands of passengers and crew members need fresh water every day for drinking, cooking, showers, laundry, cleaning, pools, and countless other activities. But how can a ship carry enough water for an entire voyage?
The answer is seawater desalination.
Rather than relying entirely on freshwater loaded in ports, many cruise ships produce a significant portion of their own freshwater directly from the ocean. Modern vessels primarily use two technologies: reverse osmosis and thermal evaporation.
How Reverse Osmosis Works
Reverse osmosis (RO) is one of the most widely used desalination technologies aboard ships. The process begins by drawing seawater into the vessel. Filters remove larger particles and other contaminants before high-pressure pumps force the water through specialized, semi-permeable membranes.
These membranes allow water molecules to pass through while blocking most dissolved salts and other impurities. The result is freshwater, while the remaining concentrated seawater—known as brine—is separated from the process.
RO systems are relatively compact and can produce substantial quantities of freshwater, making them particularly useful where space and efficiency are important considerations.
Using Waste Heat to Make Freshwater
Cruise ships can also use thermal desalination, or evaporation. In this process, seawater is heated until it evaporates. The resulting vapor is then condensed into freshwater, leaving the salt behind.
What makes this particularly interesting aboard a cruise ship is the opportunity to use heat that would otherwise be wasted. Engine cooling systems and exhaust-related equipment can provide a source of heat for freshwater production. Some marine evaporators operate under vacuum conditions, allowing seawater to evaporate at much lower temperatures.
Many vessels use a combination of desalination technologies, allowing them to produce water efficiently under different operating conditions.
From Desalinated Water to Drinking Water
Desalination is only one part of the process. After freshwater is produced, it can undergo additional treatment, including filtration, mineralization, and disinfection, before being stored and distributed throughout the ship
The water may ultimately be used for everything from drinking and food preparation to showers, laundry, and other hotel services. Cruise operators report that a large share of their onboard freshwater is now produced directly from seawater. For example, Royal Caribbean Group says approximately 90% of the freshwater used aboard its ships is produced onboard through steam evaporation or reverse osmosis.
Why Desalination Matters
Producing freshwater at sea gives cruise ships greater independence from shore-based supplies. It can also reduce the need to load large quantities of freshwater in ports, which is particularly valuable in destinations where local water resources are limited.
At the same time, desalination requires energy and careful management. Modern cruise operators are therefore investing in more efficient equipment, heat recovery, water-saving technologies, and advanced wastewater treatment.
Ultimately, seawater desalination is one of the technologies that makes long-distance cruising possible. By combining sophisticated engineering with the virtually unlimited water surrounding the vessel, today’s cruise ships can transform seawater into a reliable freshwater resource—helping keep thousands of people supplied throughout their journey.
