with how humid it was during the time of year I went, I actually didn't even use the humidifier during my trip. But yes, tap water will be fine if you can't find distilled.
This machine is going to make this trip SO MUCH BETTER!

Trust me when I say the difference of being able to breathe all night makes... really counts.
You will have to bring the cpap as a carry on if you are flying. it does not count against you for your number of carry ons. you will not be questioned about it, as TSA's see so many of these machines a day, they know it is a medical device. in fact, Cpap's go through their own little screening process as discribed here:
http://www.tsa.gov/travelers/airtravel/specialneeds/cpap.shtm
It really isn't that big of a deal and took less time for them to do than it was for my daughter to get her shoes back on.
Have fun!