I have to agree with John, I did try to control the speed via an electronic speed controller it was not that successful the speed range was limited only changing a fraction. When the speed was full the mist roared out of the funnel, the fans speed out stripped the misters ability to produce it. so I plumped for a fixed fan speed.
Maybe it's the fans I have, there ex equipment. Action make a module that you connect to an esc via a y lead. It gives the fan proportional control and even ensures it runs in the correct direction when the motor is reversed too. It costs around £30 but dies come with a fan. It's quite pricey so I suppose there is more to getting the fan's speed variable than just hooking it up to an esc. I think you need to convert the signal from the esc into a variable voltage the fan will be happy with. But I may be typing rubbish.
To answer the question about the water level, I found that my mister gives the best results if the water level is 10-15mm over its top. The mist produced decreases the deeper the water gets.
Paul