If you've ever seen photos of a cruise ship's pool, then you probably know that these are often the most crowded places on the ship. People just gravitate toward the pools on ships for some reason. Anyway, it turns out that most cruise ships have "secret" pools reserved for their staff. These are usually much less crowded since only the staff are permitted to use them, but it's also possible for guests to be invited.
Of course, a guest would have to make quite the impression on the crew in order to be invited to the secret pool, which might actually be more difficult than you'd think, considering some of the rules on board.