Could be anything. Chemicals, sewage, micro organism's, something you shouldn't drink.
Any water suitable for drinking is potable water.
I think it would depend on why the water is unfit to drink. If it is because of chemical pollutants, I'd definitely not fill my pool with it. If it is off just a little because of bacteria, a good...
Tap water is potable, or drinking, water. When it is introduced into a boiler or other non-drinking resources, it becomes NON potable