I don't think there are any proverbs in which “water cooler” occurs. Perhaps the author is referring to expressions like these (from thefreedictionary.com): water-cooler conversations: indicating the kind of informal conversation among office staff that takes place at such a dispenser. water-cooler television; a water-cooler moment. : indicating an event, television programme, etc, sufficiently controversial or otherwise noteworthy to provide subject matter for such conversations. Perhaps the author is using “proverbial” in the meaning “well known”.