I was waking with a dry mouth for about 3 weeks. It was so bad I was getting up at 3am. Waking several times in the night to drink water too.
I bought a diffuser/humidifier - one of the small, cold humidifiers. They don't heat the water. I really have no idea how they work - it's magic :) It sits on my bedside table.
I no longer have a dry mouth problem. It only uses a small amount of water, to which I add a few drops of eucaluptus oil and I find it keeps my nasal passages open. I think the problem was my nose was closing up at night causing me to breathe through my mouth and drying it out.
Hope this is of some help. :)