This is actually not quite true. We were on Infinity's Antarctica cruise from Buenos Aires in February and all they had was Coke Zero, no Diet Coke. And halfway through the cruise they even ran out of the Coke Zero. They would not even sell you a bottle or can, they would just pour part of a glass.
Then on Apex out of Amsterdam in August, they didn't have Diet Coke, only Coke Zero.
I know they're both diet beverages, but they do taste remarkably different, especially if you're used to drinking Diet Coke.