Potassium Nitrate, aka Saltpeter, is a diuretic, though no longer widely used for that purpose. It's sometimes used by those who cure their own meat and is used to make black powder.
It is not however added to food in the military or prisons, despite rumors to the contrary. While rumors still persist to this day, at least in the Navy, part of what my wife does as a PMT is to inspect the dining facilities. Quite a bit of work goes into making sure that military and government dining facilities are clean and that the food is properly cared for and fit (more or less) for humans, much more so than in private commercial establishments.