Congrats on getting your ship gig! In regards to your question it's like sz said, as long as you pass the drug and pregnancy test you should be good to go. Like any job, you can still work with pre-existing med conditions, as long as you can work. I knew people on ship with all kinds of allergies and asthma, and even a guy with HIV who worked onboard ships fo several years and had no problem at all. So no worries.
Shoppies have the bonus of being mostly off in port (shops are closed, you just might have to load or inventory sometimes) and on some lines, having privleges upon supervisor approval (going to shows, restaurants, etc). You'll likely have to share a cabin, but if it's your first contract, it's actually a great way to meet people and become acclimated to the life. Enjoy! :-)