I am now going to strike the water…and it will turn into blood
Water is cold, whereas blood is warm. There are two types of coldness and two types of warmth: a person whose primary orientation in life is material will be cold to spiritual concerns and warm to material concerns; a person whose primary orientation is spiritual will be cold to material concerns and warm to spiritual concerns.
River water—particularly the water of the Nile—signifies the coldness of materiality toward spiritual concerns. As we have noted, the annual flooding of the Nile gave the Egyptians the impression that their sustenance was due simply to the regular, orderly functioning of nature, without any need of recourse to a supernatural G-d. Such an environment fostered indifference to the notion that there is a Divine force that surpasses and controls nature.
In contrast, rainwater signifies the coldness of spirituality toward material concerns. The Land of Israel's dependence upon rainwater was conducive to keeping its inhabitants aware of their dependence on G-d's good graces for their sustenance. This awareness of G-d bred a healthy indifference toward the facade of the laws of nature's stranglehold over life.
The very first of the ten plagues, the ten stages by which Egypt was subdued, was transforming the coldness of its water into the heat of blood. Allegorically, this signifies the transformation of cold indifference to Divinity into warm enthusiasm for it. This had to be the first step because indifference can quickly lead to a drastic decline in commitment. Once this was precluded, the path was open to additional, more specific stages through which G-d's reality could be impressed on Egypt's (and the world's) awareness.
A similar lesson applies to anyone striving to leave the slavery of Egypt—the tyranny of his or her material drives and bodily desires. Our first step in this process must be to replace our a priori cold indifference to all things Jewish and holy with warm enthusiasm for G-d, His Torah, and its commandments.
We should not delude ourselves into thinking that it is possible to remain aloof and neutral, indifferent to both spirituality and materiality. If we neglect our responsibility to cultivate an energetic and enthusiastic attitude toward the Torah and its commandments, we face the specter of a swift decline into the decadence of Egypt.
—From the Kehot Chumash