Yes, there are rules, and yes, they do apply to you and everyone else on the cruise with you. So, be sure that you don’t try to hide something or lie about something to the cruise line.
If you are found out, you could be found in violation of the passenger conduct agreement you automatically agree to (if you read it or not) with the cruise line. If the ship personnel feel it necessary, you could be asked to leave the ship – and that could be in another country – where you will need to find your own way back home.
Here’s an article on USA Today’s site that can help you know what you shouldn’t hide.