You'll need to start thinking "inside out" on this one. Mileage is measured in "gallons per mile", instead of the usual "miles per gallon"

According to:
http://www.harpers.org/Transportation.html
(Just scroll down the page and you'll find it under "Facts Related to Transportation" in "1998" and then "August")

the QEII, a well-known cruise ship, goes a whopping 29 feet (U.S.) per gallon. There are 6,076 feet in a nautical mile, which means it takes that particular ship close to 210 gallons of fuel to go one nautical mile. I would be glad to put that in gallons/km for all you folks who use metric, but honestly, I'm not very good in math, and would probably get the numbers wrong. It was hard enough to figure that one out, and I could very well be incorrrect. If it helps, my conversion program says that one US mile is equal to 1.61 km. One meter = 3.28 feet. You'll have to take it from there, I'm afraid, but hey! - at least I didn't forget about you guys:)

On another search with google, I found someone who claimed that the QEII got 33.5 ft/gallon. Must have been a lighter load that day. 6076 / 33.5 = 181.37 gallons per mile. In the snippet of info google gives, you'll read that comes out to 157 gallons/mile, but that uses the standard land 5,280 feet/mile:
http://futureboy.homeip.net/frinkdocs/

If you check that out, warning: it's about a nautical mile in length. OK, maybe not quite that long, but if you're in a hurry, you could run a "Find" and search for one of the keywords. Some advice? Don't use "gallons" as that keyword. Your little search engine will be stopping five times on every page within the document. Oh joy, oh laughter. The material may be fascinating to you. It looked interesting, but ultimately, a snoozer. Far too weighty for my feeble brain.

I did google up another cruise ship that gave mileage data, but I wasn't paying attention and forgot to make note of the ship's name, or the website. This ship was smaller and sleeker than the behemoths and so fared much better on fuel - 93 gallons per mile.

I've been on lots of cruises. I was in the Navy and stationed onboard a ship for three years. We were underway probably an average of four days every two weeks. It was a mid-sized ship, not even close to the size of an aircaraft carrier, but huge when compared to a destroyer. If memory serves, the ship displaced about 9,100 tons empty and 15,000 tons when fully loaded, and was around 550 ft. in length. Anyway, I hope this is useful, and have a nice day.