A little bit of Google work can turn up who does most the crewing for cruise lines. My experience has been that the parent websites are good places to look if you are an entertainer, hairdresser, or are interested in hotel staff. Most the deck and engine positions are handled by crewing agencies, and most try to keep a single nationality among the officers.
That said, the funnest job I ever had was 2/M aboard a cruise ship. Also the worst pay. My second worst paying job paid over twice what I made on a cruise ship.