مياه is used as the "plural" of ماء, what I want to say is that when we have many origins, ressources of water, we use the word مياه instead of ماء.
example :
مياه صالحة للشرب = Drinking water : Here it can come from many origines, like rain, melted ice ...
ماء صالح للشراب = Drinking water : Here we have one ressource, which is the rain for example.
في القارورة ماء صالح للشراب = In the bottle drinking water.
I hope I made a clear explanation