Sure...tap water is OK for short periods of time (like a cruise). The problem with tap water is the minerals (and some bottled water may have this issue). If you use tap water (BTW: the tap water on the ship is perfectly fine), when you get home, just do a thorough clean/soak with 1 part water to 3 or 4 parts white vinegar.
Years ago, I had a patient who went on a cruise and didn't know water was available for his CPAP. He went without and got REALLY dry mucous membranes as a result...don't do that. If you need humidification, you need it. DH uses CPAP without water and does better than with.
BTW: he bought a Z2 Auto from Breas for travel. It's really, really small (maybe weighs 2 pounds). It would fit in a purse or backpack easily. It comes with a "French nose" for those who need humidification.