My suggestion is to look for details and for patterns. That sometimes means looking up, rather than out. Most cruise guests stand at the railing of the ship and photograph the scenery (or water) that is off to the side of the ship. Less often do we stop and look up to see if there are any interesting shapes. The shot to the left was taken on Celebrity Millennium from the Sunrise Deck looking up at the cover over the AquaSpa. I like the play between the white superstructure and the blue sky in the background. I wanted to isolate the curve of the roof so that the viewer would be drawn to the shape itself. The image reminds me of our last cruise and is an interesting study in shape and color for people who don't know that it is part of a cruise ship.
When you have some time on board your ship, see if there are shapes, colors and textures that you can isolate in your photos. The end results can be engaging both for you and others who see your photos later. Remember, less is sometimes more when it comes to photography.