Cruise ships are notorious for polluting the ocean, and Carnival and Royal Caribbean are some of the worst offenders

Royal Caribbean International

Royal Caribbean's Brilliance of the Seas ship.

• Carnival Corp and Royal Caribbean Cruises do a poor job of limiting the pollution from their ships, according to the environmental-advocacy group Friends of the Earth.
• The group graded 16 cruise lines based on how they treat their sewage, the steps they've taken to limit air pollution at ports, whether they follow water-pollution rules in Alaska, and the quality of information they provided about their environmental practices to Friends of the Earth.
• Disney Cruise Line was the only business to earn a grade higher than a C-, and no Carnival or Royal Caribbean examined by Friends of the Earth earned a grade higher than a D+.
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Carnival Corp and Royal Caribbean Cruises are the world's biggest cruise companies, but they're also among the worst at limiting the pollution from their ships, according to the environmental-advocacy group Friends of the Earth.

The group graded 16 cruise lines based on how they treat their sewage, the steps they've taken to limit air pollution at ports, whether they follow water-pollution rules in Alaska, and the quality of information they provided about their environmental practices to Friends of the Earth.

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Disney Cruise Line was the only business to earn a grade higher than a C-, and no cruise line owned by Carnival or Royal Caribbean earned a grade higher than a D+. (Every cruise line owned by Carnival that Friends of the Earth graded automatically received an F since the company admitted in June to violating the terms of its probation from a 2017 conviction for improper waste disposal.)

"We really feel as though the industry has not improved in a way that we think they should be, especially with the continued, significant profits they make annually," Marcie Keever, the oceans and vessels program director at Friends of the Earth, said in an interview with Business Insider.

Here's how the six cruise companies included in Friends of the Earth's list fared. Business Insider assigned each possible letter-grade a point value (an F is worth zero points, while an A+ is worth 12 points) and determined the average score earned by each company's cruise lines.