Why does Rashi need to add a source to prove that the order is Me'akev? The
first verse that Rashi cites ("and he shall place it in the water") should
suffice to teach that the water must be placed first and that it is Me'akev!
(a) There is a rule that in all areas of Kodshim, the procedure that is
written in the Torah is Me'akev only when the verse repeats it, adding an
additional verse. The TORAS HA'KENA'OS points out that it could be that
Rashi maintains that the preparation of the Mei Sotah is considered a matter
of Kodshim, and therefore the order would *not* be Me'akev unless there was
an additional verse. That is why Rashi points out that there *is* an
additional verse, the verse of, "The Kohen shall do...." What, though, is
Rashi's source to say that Mei Sotah is considered Kodshim?
The Toras ha'Kena'os points out that Rebbi Yochanan in the Yerushalmi says
that Mei Sotah will become Pasul through Linah (this is also the view of the
Rambam in Hilchos Sotah 4:12). He explains that Rebbi Yochanan holds that
since the water of the Mei Sotah comes from the Kiyor, and the dirt comes
from the floor of the Mikdash, there is reason to say that it has some
degree of Kedushah and that is why it is considered like Kodshim with regard
to needing an additional verse to teach that the order is Me'akev.
(See, however, Devar Sha'ul to Sotah 20a, who questions the suggestion that
Mei Sotah has the status of Kodshim based on the fact that it may be brought
to Sha'ar Niknor and is not disqualified through "Yotzei.")
(b) The TORAS HA'KENA'OS answers further that this rule -- that regarding
Kodshim a procedure in the Torah is Me'akev only when the Torah adds an
extra verse, while regarding non-Kodshim related topics a procedure is
Me'akev even without an additional verse -- applies only with regard to the
actual performance of the procedure itself. With regard to the *order*,
though, even with regard to non-Kodshim related topics, the order is not
Me'akev unless there is an additional verse to teach that it is Me'akev.
(See, however, TOSFOS 17b, DH Kodem, who implies that a verse is even needed
to touch that every part of the Sotah process is Me'akev, and not only to
teach that the order is Me'akev. Perhaps acts which are clearly *secondary*
acts and not integral to the Sotah-checking process are only Me'akev if the
verse says so explicitly.)