From the The Ocean Conservancy website located at

Ocean Conservancy Dynamic Issues Page

"Floating cities with their own zip codes, cruise ships carry millions of people each year to some of the world's most pristine and sensitive ecosystems. Largely unregulated, cruise ships also discharge huge amounts of waste directly into the water. From raw sewage to toxic chemicals, cruise ships dump huge amounts of waste directly into ocean waters, posing a potential threat to marine wildlife, fragile habitats and human health.

Cruise ship impacts have increased exponentially with the industry's dramatic growth. In 1998, 223 cruise ships carried 10 million passengers through some of the world's most sensitive ocean ecosystems. Since then, the industry has grown by an average of 10 percent annually, and is expected to bring more than 49 new vessels into service by 2005."
There is a great PDF document called Cruise Control located at this page that explains the insanity of these floating cities. Well with the way Norwalk viruses are plaguing these ships we may luck out and the Belize plans will get axed.