Whoever told you not to drink the cruise ship's water is misinformed or has not been on a cruise in many many years. All the cruise ships have water purification systems and the water coming out of your tap is as clean or cleaner than your bottled water.
If they meant the water in the ports, then yes, you do have to be carefull about that in some ports. You can take bottled water off the ship and we do bring some bottles for that purpose. You can buy bottles from the cruise line as you leave, but those are pricey. (Although if it means paying $2.50 for a bottle of water versus lugging your own bottles through airports, just buy it on the ship. We only bring our own bevvies if we are driving to the port)
Yes I heard the same thing about Coca-Cola. If you buy it here in the USA it is made with 100% High Fructose Corn Syrup instead of sugar. I heard that in Mexico and maybe Central America, they bottle Coca-Cola there and still use sugar.
So, next time I go to Mexico on a cruise, I will get a bottle or two of Coca-Cola and see if it is made with sugar or H.F.C.S.
I have tasted Coca-Cola all over the world (Africa, China, Europe). Yes, you can tell differences. You'll notice the U.S. uses the word "Diet Cola", where most other countries use the word "Lite". I think it is good, just different.
The sugar-based Coke (Pepsi too) is better than the HFCS stuff.
I remember the soda we bought in Italy years ago. Before our ship would leave the U.S. for a deployment we'd load up on Coke and Pepsi. Eventually it would run out and we'd be forced to buy from the local market at whatever port we stopped in. I don't know what they did to it at the Italian bottling plant, but it was pretty lousy. It was an acquired taste for sure!
You don't have to be on a cruise to get the sugar-based cola. Any Mexican grocery store in the U.S. will carry the original beverage. Just make sure that you check the ingredients label. Sometimes they will carry both. As far as drinking water on the ship is concerned, I have found that the taste of the ship's water supply is often superior to the bottled water sold in the states. Jim is correct in bringing a 6-pack of H2O aboard as it gives him the ability to refill the empty bottles with ship water to be enjoyed on any part of the trip.