I'm going on a cruise next week, what do I do if I get diarrhea?

Drink. If you develop the norovirus, which is showing up more frequently on cruise ships, the best advice is to drink plenty of water broth and juice, eat a plain bland simple diet, things like toast rice baked chicken, plain pasta, etc. And stay away from other travelers so you don't infect them.
Prevention is better. Prevention is a better strategy! use frequently the hand sanitizers throughout the ship; minimize contact with communal surfaces (doors, banisters, counters, etc.) that other ill individuals may have touched; wipe down gym equipment before use; pay attention to health announcements; minimize alcohol & focus on hydration; avoid undercooked or raw foods; avoid contact with sick passengers & crew.
Imodium (loperamide) Never leave home without imodium (loperamide).Anti-diarrheal medicine available over the counter and very effective too.Follow the dosing guide. No milk products till diarrhea subside.Hydrate yourself.Chicken broth.Gingerale.Flattened colas.Toast .Saltine crackers to name a few.