In Tanach, the noun מַיִם occasionally is prefixed with a ב־.
- When the noun is modified by an adjective, the word formed is בְּמַיִם, as in Bamidbar 24:7 "וְזַרְעוֹ בְּמַיִם רַבִּים", N'chemya 9:11 "כְּמוֹ אֶבֶן בְּמַיִם עַזִּים", Vayikra 15:13 "וְרָחַץ בְּשָׂרוֹ בְּמַיִם חַיִּים", and elsewhere.
- When the noun is not modified by an adjective, the word forms is בַּמַּיִם, as in Vayikra 14–15 passim, Bamidbar 19 passim, Vayikra 11:9 "כֹּל אֲשֶׁר לוֹ סְנַפִּיר וְקַשְׂקֶשֶׂת בַּמַּיִם", Ⅰ M'lachim 14:15 כַּאֲשֶׁר יָנוּד הַקָּנֶה בַּמַּיִם, and elsewhere.
Is the demarcation I note — whether an adjective modifies it or not — the deciding factor as to what form of במים is used in Tanach?
If so, then why? Why does that determine the form of במים?
If not, then what is the deciding factor?
(There's seemingly a dispute about Sh'mos 15:10 "צָלְלוּ כַּעוֹפֶרֶת בְּמַיִם אַדִּירִים". Rashbam says it means "they sank like lead in powerful water". However, S'forno (perhaps following the g'mara M'nachos 53 amud 1) says it means "the powerful sank like lead in water". Presumably, then, at least S'forno doesn't hold of my rule: he interprets the verse as not having an adjective modifying בְּמַיִם. Or maybe he agrees with my rule when פשט-interpreting verses, and is commenting only on the דרש level.)