An observant supervisor

Hi,

Ritual immersion in a mikvah requires, ideally, an on-looker who sees that the immersion is done properly, and particularly that all of the immerser's hair is submerged. The one who is immersing will not be able to determine this.

This observer must be personally observant of the laws of immersion, in order to be credible.

(Rav Moshe Feinstein, Igrot Moshe Yoreh Deah 1:105)

Have a great day,
Mordechai