Are you sure about that? A quick search for carbon per passenger cruise ship vs airplane returned several results[0][1][2] suggesting that the airplane is in fact a more eco-friendly way to travel.
I can believe those numbers saying that a transatlantic cruise would be 3-4x the emissions of a transatlantic flight. But the per-passenger-km figure isn't a realistic basis for comparison; tourists aren't trying to travel x kilometres, they're trying to see places x, y and z. The substitute for taking a 1000km cruise 5000km away from where you live is not 11000km of flying, it's more likely making two or three 10000km round-trips. And if you did replicate a cruise by flying between successive ports of call then the efficiency figures for those short-haul flights are going to be an order of magnitude worse than for a transatlantic flight.
