(figuratively, informal) Synonym of ne pas avoir inventé le fil à couper le beurre

(figuratively) to carry coals to Newcastle

(figuratively) to add grist for the mill, to bring grist for the mill, to provide ammunition for someone (to provide extra arguments in support of someone's stance)

(figuratively) to keep one's head above water

(idiomatic) throw the baby out with the bathwater

(informal) for there to be trouble brewing, for there to be tension in the air

it was a long time ago, and much has changed since; it's water under the bridge

still waters run deep

the jug goes to the well until it breaks, the pitcher goes so often to the well that it is broken at last

Alternative form of tant va la cruche à l'eau qu'à la fin elle se casse