How eco-friendly is cruising?

I've seen quite a few posts on social media recently demonising cruise ships as being particularly harmful to the environment, but how much of this is true and how much is scaremongering?
Let's break the pollution down into categories:


Air Pollution.
Sewage
Garbage
Oil

It's a fact that ships burn a lot of fuel, and that fuel is often heavy fuel oil, which is some of the dirtiest fuel there is besides coal. MARPOL Annex VI, introduced in 1997, aims to regulate air pollution from ships. Certain special areas (a full list can be found here) ban high-sulphur fuel oil from being used, and some only allow Marine Diesel Oil (which is much more expensive) to be burned. A handful of ships are fitted with seawater scrubbers to reduce the sulphur content of exhaust emissions, but these are experimental and not in widespread use, and essentially cut out the acid rain middleman by washing sulphur directly into the sea.Nevertheless, a typical cruise ship will emit more CO2 per passenger mile than an aeroplane, and if you fly to get to the cruise ship, you are adding even more emissions to the atmosphere. While modern vessels are far more efficient than the steam ships of old, they are still a major source of air pollution. For the lowest possible emissions, you may with to consider a cruise on a sailing ship. 

Disposal of sewage at sea is regulated by MARPOL Annex IV. It's probably easiest to quote the Annex at this point: "The discharge of sewage into the sea is prohibited, except when the ship has in operation an approved sewage treatment plant or when the ship is discharging comminuted and disinfected sewage using an approved system at a distance of more than three nautical miles from the nearest land; sewage which is not comminuted or disinfected has to be discharged at a distance of more than 12 nautical miles from the nearest land." Typical cruise ships are fitted with a Marine Sanitation Device, which disinfects the sewage in a similar manner to a municipal sewage plant. Ergo, it is no worse for the environment than what your city is pumping into the sea, with the advantage that the ship is dumping sewage much further from the shore! Untreated sewage may be discharged outside 12 miles, but a modern cruise ship will not do this except in an emergency. Some cruise ships are fitted with an Advanced Wastewater Purification System (AWWPS), which produces clear water (which is harmless and can be discharged almost anywhere) and a small amount of sewage sludge (which can be discharged as per treated sewage). Solid waste is pumped ashore for disposal at landfill.

Garbage comes under MARPOL Annex V. There are a variety of types of garbage, which must be separated and disposed of in different ways.
Food waste is usually comminuted and dumped at sea, since it poses no threat to marine life.
Solid waste can either be sent ashore for recycling or (if burnable) burned in an incinerator (outside of emission control areas with reference to Annex VI). Scrap metal and glass will be sent ashore for recycling. Although legally it cold be dumped at sea there is a financial incentive to sell scrap metal in port. Throwing plastic overboard is absolutely banned and companies can and have been fined for dumping plastic at sea. 

Oil is the most harmful waste stream and it is very heavily regulated by MARPOL Annex 1. Ships must keep a record of all waste oil disposed in an oil record book. Bilge water must be treated in an oily water separator and contain less than 15ppm of contamination before it can be pumped overboard. Waste oil can either be burned in an incinerator (see above) or boiler, or be sent ashore for disposal. 

Is cruising any worse for the environment than any other holiday? Well, yes. Certainly it's more harmful than backpacking/cycling holidays or a "staycation". Is a cruise from a UK port better than flying abroad? It depends on how far you're flying. A fly cruise to the Caribbean is probably the most harmful option. If you want to visit the Med, but want to avoid the pollution, it might be worth taking the train to Spain, France or Italy.