When the crippled cruise ship Carnival Triumph was towed into port in Mobile, Alabama nearly two years ago, it was a PR disaster for the industry's biggest player and an awful mess for its passengers.

An engine room fire that left the vessel adrift in the Gulf of Mexico also left the ship with only a handful of working toilets for the more than 4,200 people aboard. The "poop cruise" became a media frenzy, and Carnival was the butt of jokes for weeks.

But in a way, every cruise is a poop cruise.

The biggest ships are now the size of aircraft carriers, ferrying thousands of passengers and crew from port to port. All those buffet helpings and fruity drinks have to end up somewhere. And that means more than one billion gallons of sewage gets dumped into the world's oceans each year, much of it after only minimal treatment.

That little nugget is in the latest report card on the cruise industry from Friends of the Earth. The group ranked companies on how green they keep their ships, focusing on their compliance with coastal clean water rules, how much they run their engines to provide electricity while in port, and how extensively they treat their sewage.

"They are like small cities and they have as significant an impact," Marcie Keever, who wrote the report for Friends of the Earth (FOTE), told VICE News.

In addition to sewage, up to eight billion gallons of "grey water" from sinks and showers gets dumped overboard every year, Keever said. That's a tiny fraction of the water in the world's oceans but it's a large volume "from an industry that purports to be clean and green," she said.

"We think it's irresponsible, and think they should be doing more," she said.

Disney's four-ship cruise line was ranked the most environmentally friendly, getting A ratings of sewage treatment and water-quality compliance and a B-minus for its efforts to reduce air pollution. Princess Cruises and Holland America lines — both Carnival subsidiaries — ranked second and third. For the environmentally conscious consumer, "This is a way to make the greenest choice," Keever said.

But every cruise line received an F for transparency — a category added after the industry refused to provide information for the report, Keever said. That drove down the letter grades of all 16 companies, with top-ranked Disney ending up with a C-plus and Princess and Holland America earning Cs.

"They're not in school anymore, and we understand no one likes to be graded," Keever told VICE News. She said the industry cooperated in four previous surveys, and researchers from Friends of the Earth were able to collect enough data from public sources for this year's report.

International shipping regulations allow treated sewage to be dumped overboard as long as a ship is more than three miles offshore. Untreated sewage can be dumped overboard once a ship is more than 12 miles out. But Keever said lines like Disney — which was in the cellar in the group's 2008 survey — now get top marks "because they're doing more than what's legally required."

"We judge them on it because we think they should be doing a better job, using better technology," she said. "It's available and to a certain extent cost-efficient, particularly when the cruise industry is making record profits."

This year's report is the third straight that has put Disney on top and the third year that Princess and Holland America have rounded out the top three.

The bottom-ranked line was Japanese-owned Crystal, which received F marks for sewage treatment and air pollution reduction; the bottom three also included two other Carnival subsidiaries, P&O and Costa.

The US Environmental Protection Agency has taken steps to beef up regulations on wastewater dumping and push cleaner marine fuels, particularly around American territories in the Caribbean. Keever said all West Coast cruise-ship ports have connections that allow vessels to connect to shore power, rather than burning their heavy fuel oil to produce electricity at the pier, "So they don't have a smokestack that's running for 10 hours."

Responding to the FOTE report, the Cruise Lines International Association, said: "Any decision to not participate with the FOE on its distorted and inaccurate cruise line report card was made by individual cruise lines, not CLIA. The report issued by FOE once again fails to provide consumers with an accurate picture of the outstanding environmental performance and sustainable practices of CLIA Member cruise lines."

Follow Matt Smith on Twitter: @mattsmithatl