There actually is quite a bit of moisture in the atmosphere.
http://www.eoearth.org/view/article/150299/
http://www.climate.gov/news-features/understanding-climate/2012-state-climate-humidity
I'd say (not being a meteorologist) there is enough average humidity for the Ice King to accomplish most of the things we see him do.
Perhaps the Ice Crown uses electromagnetic fields somehow? Sort of a microwave in reverse? This might also explain Ice King's unfortunate insanity, since the crown would need to be throwing out a HUGE amount of EM right next to his head.
Of course, the crown still needs a mechanism to move huge volumes of water around. The answer must be graviton manipulation, which also explains the Ice King's flying ability.
That still leaves age manipulation. I just... I don't even... nanobots?