I believe the correct usage is “Apporter de l’eau au moulin”.

According to this site and this site in french and to this site and wikipedia, the meaning is not exactly the same.

In french it means that you help someone without knowing it, you’re giving him/her “ammunition” against you without you even realize it, while in English it means “everything is a source of profit”.

The “point of view” if I can put it that way, is one of a “3rd person” observing an argument between 2 persons for the French idiom, while it’s more of a first person (the one who owns the mill) in English.

Edit:

It seems that this site links both together.

Apparently, depending on the context these idioms vary in meaning, but they could mean the same thing.