It will probably depend on which cruise line you are using and where you are going. I've been on two Disney cruises and they initially told me that I could use the detector on their private island, until I got there. I was detecting for about 30 minutes when Disney "officials" came over and told me that it wasn't allowed. I complained and told them that I was told that it was when I booked the cruise and the manager of the island told me that they hadn't checked with him and he would have said "no" from the start. He has a detector that they supposedly use when someone says that they've lost something. His other comment was that they have insurance that pays for some of the losses and if I found the lost item, then their insurance would want to be repaid. I never figured out how the insurance company would know. They actually escorted me back to the ship with the detector to make sure that I didn't stop and look around on the walk back.
I will say that the beach and water area that I did cover was very clean. I only found 58 cents and very little trash.
Scott