The Noordam, a Holland America's Signature Class ship,.81,769 gross tonnage is considered a mid-size ship, This ship consumes approximately 57,000 gallons of diesel fuel. and 23,000 of lighter grade fuel per day. The 10 day voyage averaged 20 knots, and covering 2904 nautical miles. Again do the math.
This should give you a good idea of cruise ship fuel consumption at sea. I can only imaging what the new mega ships consume.