I used to be a stage manager on cruise ships. They should have a guitar and an amplifier for the guitar player in the show band on board - but I cannot guarantee it.
To be really safe, you could bring your guitar and a direct box, or even better, a small practice amp they could mic. Then you could also practice your chops in your cabin -- at a REASONABLE volume, of course :-)
If that is too much trouble, take your chances with the stuff they probably have on board.