How bad for the environment are cruise ships?

Correction, Sept. 1, 2009: This article originally indicated that San Francisco and Los Angeles were already providing cruise ships with shore-based power. (Return to the corrected sentence.)

Correction, Sept. 2, 2009: The article originally stated that the round-trip plane ride would emit 312.3 pounds of CO₂. (Return to the corrected sentence.)

Correction, Sept. 3, 2009: The corrected paragraph notes that under international law, bilge water must meet certain oil content standards before it can be released into the ocean. It also includes the caveat that most cruise ships have a standing policy not to dump untreated sewage anywhere. (Return  to the corrected paragraph.)