I DJ'd on ships up until recently - both for the Teen Clubs and the ships Adult Night Clubs. I worked on the 5 major cruise lines - it was a great experience and if I don't mind saying, I'm a good DJ with good mixing skills and an ever growing wicked collection of tracks. Most ship DJ's have come from a wedding dj background - not a club or radio DJ background. I am still on ships, but in another position and lovin' it.
But what makes a DJ good, is their ability to play and seemlessly mix, tracks what you want to hear, mixed with some new stuff, to keep it interesting. Guests come from all over the US, Canda as well as the world, so its great to have current knowledge of whats happening musically for the international guests onboard.
Some ships have various DJ promo subscriptions, however its not unusual for many of the discs to be missing, "borrowed" but never returned. Thus many ships tend to have a limited selection, with lots of older music . The good DJ's will not rely on the ships inventory - they will provide all their own legal music.
So take your favorite CD's with you and ask the DJ if he/she will play some tracks for you - it's your cruise!
I mean do you really want to hear the electric slide 1000 times?