Cruise ship work is cool, but you need the skills, and reading is top of those skills, SN reading in particular.
Reading charts is also required, as is a good work ethic.
In Europe you need paperwork, so a crew or transit visa is needed, this lets you enter ports of call, or a countries maritime waters, but does not allow you to stay or live there.
So depending where you work you will need the relevant paperwork before you apply.
The same applies for any medical paperwork, or H & S paperwork you may need...make sure you have it before you apply.
Much of this work is on websites, and auditions may be required, so always be honest on your abilities, skills and experience. The worst thing you can do is try and bluff your way on as you will be found out.
When i done it in the past the MD had everything in folders, so a rep of 1000 songs was common, everything from dance music `(waltz tango, American Smooth, Jive etc) to working with the shows put on, so think new show every night, to current chart songs, to classics and any genre such as Soul, Jazz, Disco, etc.
Today i believe this is iPad work, so the potential is thousands of tunes in a myriad of styles, so in theory a lot of "Click Playing" as a moving curser may be used to keep everyone in time.
Even as a bassist we could be asked to play with a piano player or guitarist, some light incidental music to people entering for dinner of events, or even play near the Captains table.
We were employed as musicians, so our employment was to play our instruments, so if not playing, practicing or rehearsing we were "off duty".
But i believe today many are employed under an entertainers contract, so you may find yourself helping out in any area covered as entertainment...and that is in theory whenever you are awake you are available to help set up shows, move scenery, baby sit kids, do showcases for passengers etc etc.
So if you google "Cruise Ship Musician Bookers" you will find lots of info and job descriptions to consider and maybe apply for.
As a rule you work for the company that supplies the entertainers, not the Cruise ship line, or any company booking the ships, but for the company that supplies the entertainers.
Good luck with it all, it is a great experience, well it was back in the late 70s early 80s, but life was simpler back then.