I would say you should learn as many types as you can. Reiki and massage can be used at the same time or separately. Shiatsu is just a different type of massage. I have a friend who is a massage therapist. All her clients get Reiki automatically (you can't NOT give it once you're attuned) and then a variety of techniques depending on what they need/ask for. The more you can offer, the better.
From what I understand, working on a cruise ship sucks. You don't get to go on shore or anything. If you don't have massages scheduled, they will make you do drudge work. There are no days off. And most cruise ships don't hire Americans.