Competition is fierce, so you will need to gather a good set of references from employers who have employed you in your specialist occupation on land. Once you have those, try getting a loist of the cruise liner companies, and try the smaller ones first. Once you have experience, you can work your way up the chain to tbe biggies, who can employ the best, and usually do, because the cruise experience for passengers is all about quality service from the crew.
Seeing as you don't mention what kind of job it is you want thought I'd add that it's worth picking up a copy of Stage. Of course you would be interested in job performing as asinger/dancer/whatever cos otherwise it wouldn't worth it,but if it that's job you're after the Stage is FULL of cruise ship entertainers wanted... however if it's as the captain, i wouldn't have a clue...