Your not supposed to run them low on water but if you want to see it work put a couple inch's of water in it and turn it on for a second and look at the waves.
You always want to use distilled water, The first time I was using it it got dirty fast so I changed it a lot then I started pre cleaning the junk off before I used the ultrasonic so the solution lasted longer. One day I took off the lid and the was some green thing growing in there! it was half the tank large blob. Then I started using distilled water and never had that problem again.