They have it on the ships, but it is in cans...which means that you pay for it. It is approximately $3 per can including the automatic tip.
Beverage station has coke, diet coke, sprite, etc...but no Coke Zero.
You are welcome to carry on a 12 pack or whatever amount you choose. You can also buy the cans in many ports and re-stock during the cruise if needed. BUT if you are going to Europe, the Diet Coke is really strange in some places.