Blood and Water

Rashi: throughout the entire land of Egypt
Even in the bathhouses, and in the bathtubs in the houses.
even in wood and in stone:
Water in wooden vessels and in stone vessels.

Baal HaTurim: and over all their bodies of water.
This phrase occurs 4 times:
(1) here
(2) "They arrived at Elim where there were 12 springs of water...they encamped by the water there (Ex. 15:27)
(3) "into the blood of the bird that was slaughtered over the spring water, " (Levit. 14:6) and
(4) "as the sun shone upon the water from afar the Moabites saw the water as red as blood. (II Kings 3:22)
These verses are associated to show that the metzora [one with a leprous-type skin disease] must place enough water in the vessel so that the presence of the bird's blood remains apparent in the water. This relates to the verse from Kings which describes how they saw the water as red as blood, so too with the metzora who should see the redness of the blood in the water. Also with the plague on the river, the blood was seen in the waters. And in the merit of the commandment fulfilled with water, the Israelites merited to find the 12 springs of water.

Or HaChayim: When Aaron and Moses will smite the river, G‑d's will would be done and the result would be the same as if G‑d Himself hit the river.

Ramban (verse 18): The water turned to blood when Aaron smote the river, but first he lifted up the rod and spread his hand over the land of Egypt in all directions. So in the sight of Pharaoh, "all the waters in the river turned to blood" and the blood was "throughout the land of Egypt." So, through the spreading of his hand, a local plague became national.

The Lubavitcher Rebbe: The water 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. 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.
...this signifies the transformation of cold indifference to Divinity into warm enthusiasm for it...
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. (Kehot Lubavitch Chumash - Shemot)

Zohar Vaeira 29: Come and see: When this blood is aroused against any nation, it is the blood of killed people, because another nation is provoked to come and kill them. But in Egypt, G‑d did not want to bring another nation to arouse blood against them, because of Israel that were living among them, so that the Israelites who dwelt in their country would not be distressed. G‑d smote them with blood in their streams instead, so they were not able to drink.

Since their dominion extended over that river, G‑d punished their dominion first, in order that their deity would be smitten first, because the Nile was one of their deities. Similarly their other deities were gushing with blood. This is the meaning of: "And that there may be blood throughout all the Land of Egypt both in vessels of wood, and in vessels of stone."

Come and see: Pharaoh ruled with water, as written: "The great crocodile that couches in the midst of his streams". (Ezek. 29:3) Therefore, first his river was turned into blood. Afterward, frogs plagued them with sounds that shook within their bowels. They came out of the River and onto the ground with high pitched voices in all directions until the Egyptians fell as if dead in their homes.

...G‑d punished their dominion first...because the Nile was one of their deities. And the secret of the matter is that all the ten signs G‑d performed originated from His strong hand and this hand overpowered all the levels of their dominion in order to confuse them. They did not know what to do. When all these levels tried to do something, it became apparent to all that they could do nothing because of the strong hand that rested upon them.