Ok... how about this? A zombie outbreak has happened while a cruise ship is at sea. The ship picks up a lifeboat of someone who's been bitten, the crew of the ship unaware that the outbreak has occurred yet. Then as the bitten person slowly begins infecting the crew, the story progresses from there. Or wait, what if the ship was at port and most of the passengers/crew were out on land when zombies started making their way onto the ship and the captain ordered that the ship pull away from port, saving the lives of those aboard but dooming those on land.
I've never been on a cruise before, but that setting sounds very interesting.