The unfortunate reality is that the RC security people on the ship who are screening luggage have the ultimate say in what is and is not allowed to be brought onboard. There is way too much inconsistency. Most of the time, as long as what you're bringing does not obviously have some sort of surge protection, they let it go. That being said, I have seen where an item is confiscated (and returned at the end of the cruise) that clearly has "no surge protection" printed on it. You can try to argue with the security folks in the naughty room but good luck getting them to change their mind. Bottom line is bring a non-surge protected adapter and take your chances. Worst case, it's confiscated and returned at the end of the cruise.
Frederick