Although gigantic ships are relatively efficient -- compared to, say, transporting massive amounts of stuff by airplane -- they do use an astounding amount of fuel. The shipping industry is working on this problem. In particular, a Norwegian shipping company and a marine energy company are partnering to work on a hybrid ship, the Viking Lady.
The Viking Lady (how awesome is that name?) already has a fuel cell installed that helps reduce its energy impact. Next year it should be getting a battery. Together, those features will mean it won't have to burn gas while it's hanging out in port -- not only saving energy, but keeping down emissions.