One important thing to consider is that you'll need to remove any "tools" from your dive bag. Most cruise lines will confiscate about 80% of your tool kit, including picks, wrenches, and screwdrivers. You might get it back at the end of the trip, might not.
@Altamira must cruise in much colder places than I do, because I think I would have passed out wearing my wet suit walking around Cozumel or Honduras in September. Actually, I might have passed out wearing it in the water on those trips. Most cruise ships have saltwater pools, so I wouldn't rinse my gear in the pool. Drying it on the lounger
right next to you will work, though, if you don't have a balcony. If you do leave it unattended on a balcony, secure it somehow because it will be windy.
Many ops in many places will happily work with cruisers and even arrange their boat schedule to make sure you get back in time. Nobody wants to end up on Yelp as the op that stranded a cruiser, so unless the boat sinks or your cab has a flat, you're probably going to make it back on time. I've only been a cruise ship diver once, diving in Cozumel and Roatan. Cozumel was great because I booked with my usual op, not the ship, and she made sure to take good care of me. Roatan sucked. I had to book with AKR through Carnival because they won't take cruisers unless they book through the ship.I learned that day why cruise ship divers have a bad reputation; for most of them, it's well deserved. I'm not saying I'm the best diver out there, but the rest of the boat sure made me look like it. And the dive masters treated us like crap. We surfaced as a group at a set time (40 minutes, I think), which isn't uncommon, but I'm used to somebody who lets me dive my tank/NDL, so I surfaced with over 1500 psi on both dives and wasn't happy about wasting half a tank.
Bring your ID, C-card, any advanced cards you want to bring (like nitrox, but you probably won't need it), credit card and cash, a cell phone if it works where you are, and I wouldn't go without my passport.
I've been on a lot of cruises and one thing that's conserved across lines is that the cabins are tiny and the bathrooms barely have enough room to turn around, so washing gear isn't easy. Just be prepared for that.