You can use this on any source of clear water. Potable just means drinking water, but the only requirement is that it is clear. This works just fine for river/creek water.
First: potable water is water that is safe for you to drink. You must assume that all water not from a tap or bottle is *not* potable. Steri-Pens are one of the very few water treatments that meets the EPA's guidelines for purity. You can use the Steri-Pen on any type of water. However, if the water is not clear you must first filter it. Second: you need purified water for personal use; your mouth is not the only point at which water enters your body. So even if you are very hot, please don't jump into the creek. Also, make sure you purify the water that invariably gets caught on the lip or rim of your bottle. Lastly, the Steri-Pen cannot desalinate water.