Hi Toby!
Congratulations, buddy. Cruise ships can be a blast for young people.
I don't know exactly how to help you because I see you are from Belgrade and I don't know how cruises are run in your area. Where will your ship depart and return from? What kind of people take cruises there?
I can tell you that Paul's book, mentioned above, is fantastic. Get it if you can.
Here it is fairly unusual for a magician to only do close up magic in a restaurant aboard ship. Most will work stage or platform shows with close up magic thrown in as an added benefit sometimes.
The best advice I can give you is this: Every ship I've ever been on has the passengers fill out a card or form rating their experience on the ship. Find out immediately how passengers give this feedback.
Once on the ship, no matter your job, be sure to ASK people to mention you on their comment card. Your employers will LOVE knowing that you made such a good impression on the guests!
Hope you have fun and make some good money!