This article from YN COMMMEEYS CELTIAGH - CELTIC NEWS MANNIN points to the environmental damage and pollution caused by cruise ships:
Cruise liners - they're a filthy business
‘Some 20 million people board cruise ships every year. And while they might return to land with fond memories of umbrella drinks and shuffleboard, they leave a lot at sea. About a billion gallons (3.8 billion liters) of sewage, in fact.’