Cynthia, I had the same problem as your husband when we went on our first cruise as, unfortunately I have found through bitter experience that no amount of locks or barriers will stop a sleep walker getting to where they want to go.
However, a friendly chat with the people in the cabin below usually does the trick. They were more than happy to put a mattress on the floor of their balcony for emergencies.
If that fails, you can't beat tying his ankle to the leg of the bed.
Good luck with your cruise and always remember the check the lifeboats.