The best way to get ideas here is to type a word into the search and see what kind of cakes come up. Try words like - nautical, ocean, beach, sea, ship, and so on. I usually find all kinds of cakes this way!
i did one awhile back, i think it turned out cute. its in my photos.
I don't know if this would work for you but what about doing a fish tank. Maybe 3 or 4 layers of lets say 9x6 cakes, you can make the top of cake black, as well as the trim on top and bottom, and all of the sides water and fish and the ocean type plants, maybe some shells at the bottom. I think that would look awesome!
I would go to the galleries tab . . . then the search tab and search for a couple of different things . . . "ocean" . . . ."nautical" . . . "lighthouse" . . . "sea shells" . . . and have a look there . . . this is what comes up for nautical . . . http://cakecentral.com/modules.php?name=gallery&meta=search&type=full&search=nautical
Thank you all for the information I will check it out and let you know how I make out.