I've never tried it cause I heard some negative things about it from people who've tried it.
From what I've heard, you need to be able to sight read, cause you'll be backing up guest singers. You'll need to know a whole bunch of standards that old people like. You'll also need stage clothes and the proper equipment.
The negative part I hear about is the money. The pay isn't too bad, but you end up spending it all aboard ship. ie, food, gambling, alcohol etc..
The accomodations are usually the worst on board and there's constant problems with theft, so I'd insure your equipment or take stuff you aren't too fond of.