I've used a CPAP for over 15 years and plenty of cruises. The tap water on the ship is just fine (in some cases depending on treatment, it can be on par with distilled water), I've never had an issue with it. Yes, Carnival has distilled water for a fee. I've seen different prices reported but since I don't use it I'm not sure of the current cost. I figure, why pay for something that most of which will end up being poured down the drain at the end of the trip. I'm not dragging it home.
JMHO/YMMV