The word מַיִם is, as far as I know, pretty consistently grammatically plural in Chumash (and, incidentally, in more modern Hebrew also). That is, the adjectives that modify it are plural adjectives, and the verbs of which it's the subject are plural verbs. For example, we have (Numbers 5:22) "וּבָאוּ הַמַּיִם הַמְאָרְרִים הָאֵלֶּה", "this cursed water will come", where the adjective "this" and the verb forms "cursed" and "will come" are all grammatically plural (which is why the verse is usually translated as something like "these cursed waters will come" even though that's awkward in English).
Why does it say "וְלֹא הָיָה מַיִם לָעֵדָה", with a singular verb, in Numbers 20:2?