It certainly can be rough, but a lot of it depends on your route. Since you start in Ft Lauderdale which is south, but will be going quite northerly to Copenhagen you will probably not experience much rough water until perhaps the last few days of your journey, with the last night being in the North Atlantic which is famous for high seas when the weather is bad. If the weather is not bad, it can be smooth as glass, however I would definitely take some Bonine along and if the captain says things are going to get rough I would take some early - before you start to feel the ship moving. If you take it too late you might have a hard time keeping it down. That's what happened to me.