Why does my budgie or finch poo in there water ? ?

Birds will not purposely go to the bathroom in their food or water dishes. Do your birds have their water dish under a perch perhaps? Make sure there are no food or water dishes under any perches otherwise the poo will land in there. It's also not sanitary for birds to be eating or drinking food with droppings in them. I'm sure you wouldn't want to eat food with your own business in them!