You will find as many diffinitions of informal, causal and formal, as the people you ask. Everyone has a different opinion. And when you are cruising you will see everything you can imagine on all nights. So good luck with this one!
Here is what I do, for what it is worth: Formal: I wear completely formal attire, i.e., long formal dress. Informal: I wear a nice dress (short) or very nice pant suit with heel and appropriate jewelry. Casual: I will wear flat sandals, pant suit that is not as casual as I would wear in port. Hope this helps!