Perhaps it is a characteristic of the type of Impatiens you have, but most should root readily - and in water. Personally I prefer to use a more organic medium such as peat and grit sand or just a gritty soil mix.
A small unheated windowsill propagator would be ideal, or simply a pot with a plastic bag over and kept clear of the cuttings. Take the cuttings nodally (at a leaf/stem joint). No rooting powder should be necessary. Use a moist rooting medium and don't water in but place the bag over or put into the propagator immediately. Give good light but keep out of direct sunlight.

Here is a link that might be useful: my website

I always pick off the bud when I root impatiens cuttings, so they focus on roots, not blooms. Other than that I just stick them in water and put them in my north window.