They do have butter on the cruise -- they have to. I can't imagine running a kitchen of that size without having butter for desserts and other dishes. You can't make escargot with margarine.
If you look on the character breakfast menu, one of the items is pancakes "served with whipped butter".
I know that we had butter on the Wonder 2 years ago; I was pregnant and very anti trans fats, and I wouldn't have eaten margarine.
Even if it's not on the table, you can ask for it.
Marcy