Its normally a bunch of different groups of people who go on cruise ships.
Each group will send someone to make a request at your DJ booth. So you may get asked for latin music, some german music, some reggae music, etc. As a cruise ship DJ you will need to have a DJ controller preferably with a bunch of songs on it already.
Most times you can get away with just playing Top 40. Most people from around the world will know this.
So as you can see, it is very rewarding and diverse working on a cruise ship. You will get a taste of everything. I recommend it.