I would think that you should look for a small water distiller. I had one many years ago but I am sure someone still makes them. At least it would be better than using tap water. I bet if you contacted the cruise line they have already solved this problem. There are probably many people who have already traveled with the same questions.
The CPAP Guardian Duffel Travel Bag is not only a stylish and spacious travel bag that is ideal for carrying your CPAP machine and equipment, but it also houses the most effective CPAP sterilizer available. The built-in Ozone Disinfector will safely and effectively sterilize your equipment from the inevitable build-up of bacteria lurking inside your system.