Diarrhea is your body's way of getting rid of something nasty. Actually, this is a good thing. If you have ingested a some icky bacteria, you do not want to stop your body from getting rid of it. Medical professionals will not necessarily want to stop diarrhea without a good understanding of what is causing it.
The usual first approach is resting the gut for 24 hours and replace lost fluids. What does resting the gut mean? Clear liquids only for 24 hours. Ginger ale is a good recommendation. Some experienced travel guides will say water or Coca Cola. Coke, weird as it seems, has many necessary electrolytes like sodium and potassium and regular Coke has sugar as well. Another plus is you can find Coke just about anywhere on the planet these days too. So, order up a couple of Cokes, take some Pepto-Bismol and wait it out. Yes, you will have to pass up on all the greasy fatty cruise foods as well as the alcohol. You can do it - it is only for 24 hours.
If diarrhea persists beyond 24 hours, then consider Imodium or Lomotil. This cruise doctor did exactly what he/she was supposed to do.
Another part of the adventure...