The “waters” question in the other forum made me look up John 7:38, which I remembered in English as “Living Waters”.
ὁ πιστεύων εἰς ἐμέ, καθὼς εἶπεν ἡ γραφή, ποταμοὶ ἐκ τῆς κοιλίας αὐτοῦ ῥεύσουσιν ὕδατος ζῶντος.
Water is singular, question answered. But where is that quote from? (And “κοιλία”?!)
Nobody seems to know the quote origin, though sometimes a mention is made of similar imagery in Proverbs 18:4: “The words of a man’s mouth are as deep waters, and the wellspring of wisdom as a flowing brook.” In fact, the quotation really does appear fairly direct, but it’s from Proverbs 18:4 LXX:
ὕδωρ βαθὺ λόγος ἐν καρδίᾳ ἀνδρός,
ποταμὸς δὲ ἀναπηδύει καὶ πηγὴ ζωῆς
Reason/the word is deep water in the heart of a man, and a river [that] springs up and a wellspring of life.
Much much closer. The LXX brings in the “in the heart of a man” imagery, and interprets the Hebrew as a parallel structure, with ποταμός referring to the same, easily read as also coming out of the man’s heart.
Notice that this image also explains the somewhat bizarre κοιλία in John 7:38, leading as it does to immediate bathroom imagery. (What rivers normally come out of a κοιλία? Glad that translators always fix this by glossing “heart”. Good job, guys.) In the LXX Proverbs 18:4 image (where we have καρδία), κοιλία comes to mind as a natural descriptor for underground spaces. So it can be read as a part of the metaphor in John 7:38, the mental image being something like “rivers of living water flow out of his aquifer.”
Still though…I personally would have been more comfortable with the Greek version of John 7:38 had our Lord said “ἐκ τῆς καρδίας αὐτοῦ” and dismissed κοιλία to the sewers. I expect that a lesser thinker – say Plato or Aristotle – would take an emendation here, based on the strength of the LXX source, and the similarity of the two words.