And their grain offerings and their libations for the bulls, for the rams, and for the lambs, according to their number, as prescribed.
Torah Temimah: Taanis 2b
English: It was taught: R. Yehudah ben Beseira says, it is said on the second [day] “and their libations,” on the sixth “and its libations,” and on the seventh “according to their rule” — mem, yud, mem spell mayim; from here a hint to the water libation from the Torah.
Gematria / letter-hint for water libation.
Sukkos water libation is anchored in the extra letters of the musaf verses.