while some of them may not care enough of environment, majority of them do, no less than any of the backpackers.
Cruiseship passengers have spending power - they buy souvenirs, they drink, they eat, they spend a lot on local tours. Backpackers hardly help anything for the local economy, because they are all on very tight budgets.
Do some research on this subj before you come to that conclusion that you dont do Caribbean cruise because "cruiseship passengers destroy environment".