Water is matter without form. It represents plurality – going with the flow – unrestricted matter – freedom to do whatever one desires. Egypt, who worshiped the Nile River, was an entire society dedicated to the pursuit of pleasure.
Exodus from Egypt represents the passage from one reality to another, from matter without form (aka pleasure seeking) to matter shaped accordingly to Torah guidelines.
The focus on the doorposts ( HKBH commanded that the Jews place some of the lambs’ blood on the doorposts of their houses as a sign that Jews lived in those homes) where we hang mezuzot represents the passage from one reality to the other: from the home to the street, from the area where matter has a precise shape to a place of infinite possibilities.
Normality today is, unfortunately, like Egypt, “the street”, where “anything goes.”
Judaism is comparable to “the home,” where one works according to values and Divine law.
The mezuzah represents the boundary between the two.
(Summary of Rabbi Yaakov Asher Sinclair’s teaching)