I think basic stealth vaping is in order. It's not like your luggage has to go through an NSA screening? (Please tell me I'm right on that...) If you carry it on and no one notices the vape... I don't see a problem coming. They must have far, far bigger issues with desperate smokers doing the nasty. And even then do they throw them overboard if they get caught? (Oh wait, this is Carnival we're talking about...)
I vape in all sorts of places where it's, technically, a no-no. In stores, on the subway platforms. (Underground passages too.) I'm not looking for an "it's my right!!!" fight when I do it. But in two years - never an issue. Then again, I is sneaky!
Have fun. Be sneaky. Vape one on the upper deck for me.