I don't think these ships run on steam. They use gas turbines, which produce hot air and not steam, which generate electricity which goes into electric motors connected to the propellers.
How 16 ships create as much pollution as all the cars in the world
http://www.dailymail.co.uk/sciencetech/article-1229857/How-1...
For those who don't follow the link, the 16 ships in question burn the sludge that's left over after refining all the cleaner burning bits out of petroleum. So it's not so much a "quantity of fuel burned" problem as it is that they're burning the worst possible thing you can burn, as far as this sort of pollution goes.
It's quite unpleasant stuff but you have to remember to factor in just how much stuff its moving across the planet.