Royal Caribbean’s new Allure of the Seas ship carries more than 5,000 passengers and features four pools, 10 whirlpools, a zip line over a garden-filled “Central Park”, an ice skating rink, 3D movie theatre and the show Chicago: The Musical. It also has the best wastewater treatment system a ship can offer.

That new treatment plant is not listed among the ship’s enticements on Royal Caribbean’s website, but it will earn the cruise line a higher grade on the Cruise Ship Environmental Report Card, a project of the activist group Friends of the Earth (FOE). Marcie Keever, the group’s oceans and vessels project director, said updating a sewage system is one of the biggest improvements a cruise ship can make to lessen its negative effects on the environment. And a vessel carrying thousands of passengers has a sizable impact: a large ship can generate 210,000 gallons of sewage and one million gallons of “graywater” from showers, laundry and cooking per week, according to the Environmental Protection Agency (EPA).

Keever said that in addition to water pollution, the group is concerned about air pollution from burning dirty fuel --  at sea and while docked in port. The ships need energy to power all those amenities, even when they are not moving,  but any cruise ship can be retrofitted to plug in to land-based power systems, thanks to a technology called “cold ironing”, which eliminates the burning of fuel for electricity.

“Some lines are doing the right thing,” Keever said. “Others are doing business-as-usual type of activity and not doing the big things to reduce their impact on the environment.”

It can be difficult to find information about these technological improvements, and that is one reason FOE launched its report card, Keever said. For 2010, the overall grades for cruise lines were not that impressive – the highest score, given to both Holland America and Norwegian, was a B minus, while Crystal Cruises was the only one to get an F. Tourists can also search the report card by destination or by individual ship – as opposed to the full line – and some ships that are decked out with the latest eco-technology received an A or A minus from FOE.

The Cruise Lines International Association (CLIA), an industry trade group, said its 26 member lines, including Royal Caribbean, Princess Cruises and Carnival, have taken several steps, such as having senior staff in charge of environmental programs. “We meet – and in some cases exceed – the many international, federal and state environmental regulations that apply to cruise ships whether at sea or port,” CLIA says on its website. Critics have said the regulations are not sufficient, and they can be tough to enforce as ships move in international waters. The EPA, for example, is considering updating its standards for ships discharging waste in Alaska.

CLIA has also partnered with Conservation International, a non-profit that works with businesses and organizations, to promote best environmental practices. Ships are adding recycling programs, low-flow shower heads and even solar panels. Ships may also offer a behind-the-scenes tour of their lesser-known perks if a passenger inquires.

These floating cities at sea bring economy-boosting tourists when they pull into port, but that can also spell environmental trouble. The town of Skagway, Alaska, is now on an EPA watch list because its overtaxed wastewater treatment plant has the potential of violating the Clean Water Act. Skagway’s population of 900 skyrockets to 14,000 on days when cruise tourism is in full swing, according to Alaska's Juneau Empire newspaper.

The town, just like some of the cruise ships, is upgrading its treatment facility.