Well I'm studying ship design in college right now, and I plan on going into cruise ship design when I'm out in the real world.
That said, cruise ship propulsion systems differ greatly from that of cargo or military. Engines are just huge diesel-powered electric generators. For propulsion, cruise ships use Azipod technology. Azipods are pods that hang underneath the back of the ship, can be turned 360 degrees, and have the propeller attached to the front of them, turned by giant electric motors. (http://en.wikipedia.org/wiki/Azipod
). All the systems that run the ship are powered solely by electricity.
We all know how strong the apparent wind feels when we stand outside in the front of a ship plowing through the ocean at over 20 knots. Why not then put wind-powered turbines to create electricity out of that wind? Whatever electricity is created by the wind is that much less that the engine needs to create, and that much less fuel the engine needs to burn.
And if somehow solar panels could be installed, that would be even less electricity (during the day at least) that the engines would need to make.
Right now, I don't know how practical it might be, but give me a few years until I finish college and I could get back to you on it.