Yes it is true. Sugar is also used in limited production runs of U.S. made Coke for Jewish holidays, but this is only sold in limited areas (like NYC). But rather than go through the hassle of bringing back Coke from Cozumel, if you have hispanic grocery stores in your area they probably have the Mexican Coke. Ours do up here in north Alabama. There apparently is a robust business along the border, where people go to Mexico and load up on soda and then re-sell it back here in the states. I also read that Coke has started producing sugar-based soda again in the southwest U.S. - they are not happy with what they see as smuggling and decided to cut in on the business.
The Mexican soda has a lighter mouth feel and a better thirst quenching quality than the fructose stuff.
As for the cruise lines minding...well, it depends on the line and the quantity of soda you try to bring aboard. They may even hold it for you until the last day of the cruise, like they do with booze.
It is sold as Coke. You have to look at the label though, to see if sugar was used and if it was bottled in Mexico. Coca-Cola can't do much to stop it since it is lawfully bottled with their trademark in Mexico, and it just so happens to make it's way back here. Of course if a U.S. distributor started selling it, they'd have the right to take action against that distributor. You can also find it in the internet, but it isn't cheap. Actually, I like another sugar-based cola that is made in New Jersey - Boylan's Cane Cola. Very interesting taste, with a sort of hint of cinnamon.
Some people may disagree with me on this but if you order a fountain Coca Cola in Mexico... ORDER IT WITHOUT ICE! I know Cozumel is somewhat more modern than some Mexico cities but after the hurricane, their has been a nice upsurge of Montezuma's revenge. If you don't believe me, I have a wife that will go ahead and write you a prescription to Cipro just in case.