I am the editor here, but I noticed this thread has been viewed over 100,000 times, so it is pretty popular.
I tend to agree that there are few truly private places on cruise ships largely because of security cameras, but also because they are not designed to have hidden places - they are designed to maximize the view of the ocean with lots of open railings and glass everywhere.
If you really feel compelled to have this thrill you might go to one of the top decks in the middle of the night and possibly find a stack of deck chairs that you can hide behind.
But in all truth - you need to know that will only protect you from passers by. Modern cruise ships do have literally 1000s of security cameras, most of them focussed on the open decks, looking for people getting too close to going over the side. It is mandated by law that ships must have these cameras, and that they record.
So - even if you think you are getting away with it - in reality you are probably just making a porn video for the crew to enjoy the next day.
If that is your thing - I am not here to judge you. I am sure you wouldn't be the first to do this.
On the other hand - if you get a cabin with a balcony you can always do it out there and get the same thrill. I don't think the camera will pick you up unless you happen to stick an arm or leg through the railing.