Parashas Metzora · Passuk 15:13
Chabad.org — English translation + Rashi
Torah Temima points to Kiddushin 25a.
English: “And he shall wash his flesh in water” — Ulla said: All agree regarding the tongue for immersion that it is like something hidden; what is the reason? Scripture said “and he shall wash his flesh in water” — just as his flesh is external, so everything [required] is external.
External body for immersion
Chazal do not require water inside the mouth or tongue protrusion for a valid immersion of the zav.