I have had my Phatom 4 confiscated in 2016 on the Celibrity Millennium when I returned after an early morning video shoot at Yokahama. It was 6:30am in the morning with no one else in the ingress line up. My Phatom 4 was in a backpack. The security person took his time to examine my backpack and check his rule book. It was returned to me at the end of the trip.
I just came back from another criuse in May/June 2018 on the Celebrity Eclipse in Ireland/Iceland with my Mavic Pro stored in my DJI combo leather pack. I took it with me on every port and they did not take it from me when I returned to the ship. Either they don’t care anymore, or they did not identify it as a drone. I took all the props off and I made sure the Mavic is in the upright position when I put it in the scanner. And I always returned to the ship when there is a long line up.
Another thing that I did is that I registered with Irish Aviation Authority (IAA) and I had an IAA sticker with registration number on the top of the Mavic. I was prepared to make my case with their security that my drone is legal to be used in Ireland. It never came to that.
I ran into another passenger who also brought his Mavic Pro with him and also did not have any problem.
My recommendation is to stay low profile, enter the ship with other people. Take the props off and put the drone in the upright position when entering scanner. Be prepare to tell them it is a video camera if they ask. And most importantly, do not use the drone on the ship. You will definitely ruin it for everyone who wants to take the drone on their cruise trips.