Hi Kyla,
Great question! There is a self service 24-hour drink station on every Disney Cruise Ship where guests have unlimited access to soda, coffee, tea, hot chocolate and water (dispensed water, not bottled). Coke products are featured onboard, so I would guess that some people that prefer Pepsi products may opt to carry their own onboard.
Personally I prefer bottled water, but for the sake of convenience I purchase one of the
water packages from the cruise line. They will deliver the water directly to your stateroom. Some guests would rather purchase water prior to sailing and carry it on with them.
The water, beer, soda packages are available prior to sailing, and can also be purchased on embarkation day. There will be several stations set up throughout the ship offering these packages, and the beverages will be delivered later that day.
It's important for me to mention that any beverages you bring with you onboard must be sealed and carried in your day bag. Beverages are not permitted in checked luggage.
I hope this helps!