Interesting subject. Being an engineer geek, I've done several behind the scenes tours during cruises. I learned that everything on the ship runs off the numerous massive diesel generators, even the propellers are turned by electric motors. Based on this I assumed that the generators ran a constant optimized speed and were quite efficient. Doing a little searching based on this thread, I found that the cruise industry has a long way to go on pollution.

Some ideas are in the works, like running on natural gas while at port or fuel cell technology but these ideas are a long way off.