Our Gemara discusses the concept of Hashaka. Hashaka, which etymologically is related to both kissing (neshika) and drinking (lehaskos), is the idea that two containers of liquid can join globally as one item. That is to say, if you had water that was impure inside a vessel, if you submerged the vessel in a Mikvah just to the rim, where the water in the vessel and the water in the Mikvah are momentarily joined, the water now becomes part of the Mikvah and is purified.
There is a practical use for this principle which is easier to understand if you can appreciate that the Gemara and Mishna was written in a time where there wasn’t clean running water. Imagine you had a bucket of fresh clean water but it became impure. The Mikvah water was not so fresh or desirable to drink. Thus by momentarily joining the waters, in reality the waters did not mingle much, but halakhically they fully joined, and now you have pure, and hopefully clean water as well.
The mystics couldn’t resist the symbolic potential about the idea of a momentary joining creating purification on a total scale. See for example Resisey Laylah (19), and Shem MiShmuel (Beshalach 7). Shem MiShmuel (Yom Hakippurim 5) offers An interesting idea about Yom Kippur. The mechanism by which the sacrifices of Yom Kippur achieve forgiveness is not through the typical repentance process, but instead is achieved via a direct attachment to God, analogous to this Hashaka principle.
Once on the topic of joining waters, I will share an idea based on a Gemara and Maharal. The Gemara Sotah (2a) declared that the miracle of creating a match between husband and wife is as “hard” for God as the splitting of the Red Sea. The Maharal (Be’er HaGolah 4:17) explains this in a deeper way via his understanding of physics. Generally, it takes equal force to bond something that is supposed to be seperate as it takes to separate something that is meant to be bonded. Water bonds together universally and resists individuation while humans universally are individuals and resist bonding as one. Thus, the supernatural miraculous splitting of the Red Sea is equivalent to making a marriage. Just as breaking water apart is unnatural, so too is joining humans together!
So far that is the Maharal. I would like to add a different spin. Water has a unique quality in that it appears like one single mass, no matter how big or small it is. That is, a whole bunch of single tiny droplets separate by themselves are each self contained masses of water. You don’t see any torn or rough edges delineating where one droplet broke off from another. On the other hand, if you take a whole bunch of droplets and join them together they will look like they always were together.
Perhaps, this is how the Gemara compares the splitting of the Red Sea to marriage. Even though the Red Sea was violently split and each group of water stood apart from its counterpart, the minute the water returned back to its place you wouldn’t ever be able to tell that it was divided. Yes, it is true that humans are individuals and resist joining together. However, once they do join together, it can become as seamless as waters that were once separated and became re-joined.
Translations Courtesy of Sefaria, (except when, sometimes, I disagree with the translation .)