The water needs to be deep enough so that your feet do not touch the pool bottom when you are running in the upright position.  We typically say it should be about 5 inches shorter than you, so if you're 5'5", it will need to be 5'.  If you are 6', it will need to be 5'7".  If you are within a couple inches, you can purchase a pull buoy to raise you up a little higher.