They'll be happy to
. Just order a Diet Coke with lime. In fact, if you use the same bartender very often, they'll remember what you like.
I've never bought the soda card. Like you, I've heard that the fountain DC can be flat. I prefer to buy mine by the can, but if you drink a LOT
of DC it might be worth trying the card. If it seems flat at one bar, try another.
Have a wonderful cruise!