I recently took my service dog on a cruise to South America. The crew setup this box for my dog to poop in... My dog didn't poop for a week straight. I forgot to get a stool sample from my dog checked out for parasites before we left, and 0 training for the box pooping.
Recommendation for cruising with a service dog:
1. Triple check that you have EVERYTHING you need before you leave. Not just what the ship needs, but what the other countries need to allow your dog to step on their soil.
2. Train your dog to poop in a box like the image below. It didn't take too long when we got home, about a week of training.
Edit: When we finally departed the cruise ship, my dog took a dinosaur crap... I was trying to get him to eat extra jumbo hot dogs from the buffet on the cruise so he would go potty, but it made his suffering worse.