Hi,
I work for Holland America. From what I hear, they're pretty much the best paying line for tech jobs, although I've heard a lot of really good things about Princess, too - especially if you don't mind sharing a cabin. I've never heard anything good about Carnival and I know a lot of people who I work with now left Royal Caribbean.
You probably won't be able to get a general "stagehand" position because those are usually filled by Filipino's on most lines - they're a lot cheaper to hire than any of us. Cruise ships aren't really a place to go to build up experience, you need to have good grasp on your job there because things are often going to go crazily wrong, while you're at sea and you are the onboard "expert" in your area. That said, working as a stage manager I have built on all my tech skills because I take the time to hang out with each of my technicians while they work on their equipment and ask them to teach me about it. But if I hadn't already known how to stage manage when I started working there, I would've been screwed.
You also need to be prepared to work 7 days a week for months at a time (on the plus side, you do get 1-2 months off at a time between contracts). You'll have to answer to Cruise Directors who often don't have the first clue about running a theater. And like I mentioned before, when things break, you're pretty much on your own - if it can't be fixed by your or your techs, it becomes a big hoopla to get outside contractors onto the ship to fix the equipment and if it's not absolutely show stopping sometimes it takes a while to get anyone there.
You also get to see the world and meet a lot of cool people while working in a theater instead of flipping burgers somewhere.
I hope that helps a little.