You can contact reservations to send you a medical assistance needed form. Fill it out and fax bac, and when you get on board they will have a gallon of distelled water as well as an extension cord. My husband uses a CPap and we do this on every cruise. There is no charge for this service and so far, it has always been there for us!
I was on the Liberty of the Seas last week and had a gallon of distilled water delivered to my cabin free of charge. They even asked me if I needed an extension cord. I always pack an extension cord in my C-pap carrying case.