Water stains on plaster are a common annoyance of old houses. They have to be primered before they can be painted over.
I bought the most widely used primer paint, which is also the cheapest, Kilz II. I tried it and it didn't work. The stain went through it like prunes through a baby. So I went back and got the Glidden primer. It is a few dollars more but one can see right away that it is much better paint.
It doesn't work either but you feel much better about yourself after you use it. The stain went through it like the government through money. When it is dry I will go back and get oil-based primer.