I'm not much of a soft drink person so I wasn't paying very close attention; but I don't recall seeing any caffeine-free diet drinks. I know they had Coke products, and as Krysta stated, Diet Coke; but no caffeine-free diet sodas. Sprite has no caffeine but they don't carry the diet variety.
A friend of mine drinks only caffeine-free Diet Coke and he always takes his own, except when he cruises Crystal. They actually lay up a supply for him when he asks in advance. Now that's what I call service!