Cruise Ship Water: Where Fresh Water Comes From and Where Sewage Goes

Cruise ships make most of their own fresh water by desalinating seawater. A large ship carrying thousands of passengers and crew can produce hundreds of thousands of gallons of drinking-quality water a day while sitting in an ocean of undrinkable salt water. This guide follows the water from intake to tap, then follows the used water back out through the ship’s treatment plants.

How cruise ships make fresh water

Ships use two main methods, and many carry both:

Evaporators (distillation). Seawater is heated, often with waste heat from the engines, so it turns to vapor and leaves the salt behind. The vapor is condensed back into very pure water.
Reverse osmosis. Seawater is forced under high pressure through fine membranes that block salt and impurities while letting water through. It is generally more energy-efficient than evaporation and is common on newer ships.

Either way, the output is very pure and has to be conditioned before it is fit to drink. Minerals are added back for taste, the water is filtered, and a disinfectant, usually chlorine, is added so it stays safe as it moves through the ship’s piping.

Why ships make water instead of carrying it

Cabins, galleys, laundries, pools and crew areas use a huge amount of water every day. Hauling that from port would cost weight and space and tie the ship to frequent refills. Making it on board from the sea the ship is floating in is far more practical.

Ships still take on water in port, called bunkering, especially before long stretches at sea or when the plants need maintenance. Water loaded ashore has to come from an approved source and be delivered through dedicated, sanitized hoses and connections, then treated like any other potable water on board. Desalinated and bunkered water is held in large tanks, so the ship has a reserve if a production plant goes down.

How cruise ships treat and test drinking water

Pure desalinated water is not what comes out of your tap. A typical potable water system on a large ship works in stages:

Production or bunkering. Water is made on board or loaded in port from an approved source.
Conditioning. Minerals are added back, since distilled water is flat and can corrode pipes.
Filtration and disinfection. Filters and a chlorine-based or similar disinfectant keep the water safe as it sits in tanks and travels through miles of piping. Some ships also use ultraviolet treatment.
Distribution with backflow protection. Valves and air gaps stop water from flowing backward from sinks, galleys or laundries into the drinking supply.
Monitoring and testing. Crew check disinfectant levels and water quality at multiple points on a regular schedule and keep records, which inspectors review.

Pipe systems also have to be managed against bacteria such as Legionella, which is why ships flush, disinfect and maintain hot water systems and pool and spa equipment. The CDC’s Vessel Sanitation Program inspects these areas.

Ships also manage how much they use. Many ships have vacuum toilets and low-flow fixtures, and laundry and galley practices are designed to cut waste. Water production, storage and consumption are tracked closely, because running short at sea is not an option.

Is cruise ship water safe to drink?

Yes. Cruise ship tap water is treated, disinfected and regularly tested, and it is considered safe to drink from the cabin tap. Ships sailing to or from U.S. ports with foreign itineraries are covered by the CDC Vessel Sanitation Program, which inspects potable water systems as part of its unannounced inspections. Water handling, disinfectant residuals and backflow protection are part of what inspectors check, and inspection results are published by the CDC. For how the scoring works and which ships have had outbreaks, see cruise lines ranked by norovirus outbreaks.

Cruise lines have every reason to keep water clean. A waterborne illness on a ship with thousands of guests is a serious problem, so quality is monitored continuously.

Plenty of guests still buy bottled water or a beverage package. The usual reasons are taste (treated water can have a faint chlorine note), convenience when heading to the pool or ashore, and habit, not safety. If you do not like the taste, fill a reusable bottle at an onboard water station, add lemon, or use a small filter bottle. Be more careful with tap water ashore in ports, where local standards can differ from the ship’s.

How do cruise ships dispose of sewage?

Cruise ships do not simply dump waste into the sea. Wastewater is separated into streams and treated on board, and what is discharged is released only under maritime and national rules. Solids and residue that cannot be discharged are stored on board and offloaded to shoreside facilities in port.

Blackwater and graywater

Blackwater is sewage from toilets and medical facilities. It is the most heavily regulated stream and gets the most intensive treatment.
Graywater is used water from sinks, showers, laundries and galleys. It is less hazardous, but it is still managed carefully, and some regions restrict it.

Keeping the two apart lets the ship treat each appropriately.

Advanced wastewater treatment

Modern ships use treatment plants that work like a compact land-based sewage plant. They typically combine biological treatment, where microbes break down organic material, with filtration to remove remaining solids and disinfection, often with ultraviolet light, before discharge. Many newer ships use membrane bioreactors that produce high-quality effluent. Performance varies by plant and by how well it is operated and maintained.

Discharge rules

Global rules come from MARPOL Annex IV, the International Maritime Organization’s regulations on sewage from ships. In broad terms:

Sewage treated by an approved plant may be discharged under the Annex’s conditions.
Untreated or only comminuted and disinfected sewage must be discharged much farther from land, and only while the ship is underway.
Some designated special areas, such as the Baltic Sea, carry stricter requirements for passenger ships.

National and regional laws add to that. In the United States, the EPA regulates vessel discharges under the Clean Water Act and related vessel discharge programs, and states can add their own limits. Alaska has long had some of the strictest cruise ship wastewater requirements, and discharge is banned or tightly restricted in sensitive areas such as marine sanctuaries and many coastal zones. Check the current rules for any specific region, because they change.

Other waste streams

Sewage is only one of several waste streams. Oily bilge water is processed through oily water separators and monitoring equipment before any discharge, and MARPOL Annex I sets strict limits on what can be released. Sludge from treatment plants, food waste, plastics and other garbage are handled under separate MARPOL annexes, with much of it stored on board, incinerated or landed in port for disposal. Lines publish sustainability reports describing their practices, and the official sites of lines such as Royal Caribbean and Carnival are a good starting point.

The strictness has a practical reason. One large ship carries thousands of people generating waste every day, and poor handling would damage the marine ecosystems and destinations the industry depends on.

Part of our How Cruise Ships Work hub.