Rabbi Aharon HaLevi of Barcelona, Sefer HaChinuch, Mitzvah 173- As for the reason that water purifies all [spiritual] defilement, I would suggest according to the basic level of understanding that a man should see himself after immersion in the mikvah as if he was created at that moment, just as the world was filled with water before man was created, as the verse states (Gen. 1:2), “and the spirit of G-d hovered over the face of the waters.” He should place within his heart the image that just as his body is renewed, so his deeds will be rejuvenated for the good, and they will become worthy, and he will be meticulous in following the ways of G-d.
Rabbi Aryeh Kaplan, Waters of Eden- One of the most obvious qualities of water is the fact that it is a liquid. Indeed, in a sense, water is the primary representation of the fluid state. In many instances, when we speak of “water” we are actually speaking of the fluid state in general. Therefore, the spiritual counterpart of water is very closely related to its property of being a fluid. But what is the main difference between a fluid and a solid? What special property does a fluid have that does not exist in the solid-state? The main difference between them involves change. If only solids were to exist, there would be no change at all. The world would be a dead, airless body in an unchanging frozen state. For change to be possible, the fluid, as well as the solid-state must exist… [W]e see that water itself represents the change and flow toward G-d’s goal. When a person immerses himself in a mikvah, he immerses himself spiritually in the basic concept of change itself. Man’s ego represents the element of his permanence, and therefore, when he is totally immersed in the concept of change, his ego is nullified. Thus, when he emerges from the mikvah, he is in a total state of renewal and rebirth.