I'd go with a Pirates of the Carribean or Little Mermaid theme, seeing how it's for a cruise!
I made my BFF a pirate cake a couple of years ago. It turned out really well! I frosted half of the cake with "sand" colored frosting, and the other half with ocean blue and white frosting swirled together to form a "sea". Then I covered the "sand" half of the cake with light brown sugar...it looked real! And it's fairly easy to find "pirate" cake toppers...I got a little plastic pirate ship that came with a tiny treasure chest, palm trees, and pirate figures at a cake decorating supply store. For the finishing touch, I put gold foil-wrapped coins all around the edge of the cake.
Little Mermaid would be easy; they don't make the cake pans for that anymore, but the Ariel-shaped cake pans do turn up on Ebay and at the thrift store quite often.
Happy decorating!