This one is kind of an open secret, but cruise ships have cameras in nearly every public area on board. While rare, crimes are committed on cruise ships, so it's important that crew members, security, and authorities have something to look to when trying to figure out exactly what occurred and who committed what crime. Without cameras recording nearly every square inch, that would be impossible on board a ship.
However, don't fret too much because these cameras aren't placed in more private areas, such as a bathroom or cabin. That said, most ships do cover the balconies for obvious reasons.