If you check a box of soda or case of water with an airline, it's a checked bag (and they might not take it because the packaging is too flimsy). If you carry it on, it counts as your carryon item.
I'm always fascinated with what people try to carry on. I saw a list one time compiled by the flight attendant's union. My three favorites were a full double bass fiddle, a small refrigerator and a car door. A case of water would probably make the list too.
Why not just buy the stuff when you get to the embarkation city? I know. You're taking the transfer right to the ship and won't have a chance. Anyway, like somebody said, the water aboard ship is just fine.
Also, many Western Carib runs use Key West as the first stop. If that's your case you can spend your time in Key West buying soda. There's a grocery store very near the piers. Just ask the locals.