The Water of the Conviction of the Lord
In my last (substantive) post I discussed a few things from the Protoevangelium of James 16, 17, and 18. I must give props to Nathan Stitt for linking τὸ ὕδωρ τῆς ἐλέγξεως κυρίου, "the water of the conviction of the Lord", with Numbers 5:11-31. I think you’re right. Note this quote from that passage:
ἐν δὲ τῇ χειρὶ τοῦ ἱερέως ἔσται τὸ ὕδωρ τοῦ ἐλεγμοῦ τοῦ ἐπικαταρωμένου τούτου.
Now in the hand of the priest will be the water of this reproach which curses.
You can see that ἐλέγξεως and ἐλεγμοῦ are not the same word, but they are from the same root. And since the contexts are the same, I would have to say that the practice in Prot. Jas. is the same one from Numbers.
Thanks for pointing that out, Nathan.