Cruises are enjoying a boom fuelled in part by hordes of Chinese tourists taking to the high seas, but green groups warn a wave of hulking new liners may cause environmental devastation.Gone are the days when cruising was the preserve of retirees, who would while away the hours lounging in deckchairs, playing bingo or taking part in formal dinner-dances.Modern liners resemble floating, futuristic cities capable of carrying thousands of passengers, where robot bartenders serve drinks and passe