Cruise ship pollution is a subset of general ship pollution of the air and water that is a growing problem due to the increasing globalization of industrial, commercial and cruise shipping. The pollution is not limited to the open seas, but affects coastal and inland biomes as the ships reach ports and travel along freshwater routes.
The main sources of ship pollution are exhaust from the massive fossil fuel burning engines, ballast water discharge, cargo discharge, waste discharge and noise pollution.
The ballast water alone accounts for transfer of alien species, unwanted organisms, hazardous waste, and algae blooms between distant areas and ports. Ballast water is sea water that is pumped into compartments of the ship's hull to allow the ship to manage yaw and pitch or to balance the weight and load of cargo. This sea water can come from any waterway and can be released in any other waterway that the ship accesses, transferring any number of problems.
The bilge systems and waste discharge, consider that it is the human, animal and other waste of the ship. When discharged, it is like emptying a city dump and a city sewage treatment plant into the open marine or aquatic environment. There are human, plant and animal wastes, bulk garbage, microbes, chemicals, algae and other problems that are injected anywhere in the world of seas, rivers, lakes and inter coastal areas.
For exhaust, consider the size of the biggest ships and the gas and oil that they burn. It is said that the 15 biggest ships emit more exhaust than all of the cars in the world, or of about 760 million cars. This is because the engines generate over 100,000 horsepower and there are multiple engines per ship! Even though the engines are said to be highly energy efficient, they are running all of the time and they use heavy fuel oil, not the lighter and more efficiently burning gas that cars use. For cruise ships, the engines may be smaller, but they certainly contribute the equivalent pollution of many, many cars.
Cruise ships have a lot of cleaning fluid, photo processing chemicals, paint and other pollutants that are unique to the leisure and luxury industry, and tossing the stuff overboard has been a problem that seems to go on and on. Think of a city that has thousands of people and that unloads it's sewage, garbage, electronics, industrial chemicals, cooking oils, and used petroleum goods into the nearest waterway, rather than handling it according to the best practices of recycling, waste disposal and treatment.
Finally, cargo discharge can consist of any human, animal, hazardous, toxic, bulk, or problematic thing that a ship can carry. In one case during the 1980s, the Navy was embarrassed by a case where a ship's captain ordered tons of furniture simply tossed over the side.
The global future of the cruise ship contribution to pollution is bleak, but individual nations and, within the United States, states are charged by the Clean Water Act to take more stringent measures to put requirements into law. Despite a few international and UN treaty efforts, there is the issue of exemptions from such environmental laws as the US Clean Air Act as well as the frequent absence of proper disposal facilities at ports, especially in the Caribbean and in poorer countries.