Exodus 8:16

And the Lord spake to Mosheh, Arise in the morning, and stand before Pharoh: behold, be goeth forth to observe divinations at the water, as a magician; and thou shalt say to him, Thus saith the Lord, Emancipate My people, that they may worship before Me;

In the case of certain plagues, [i.e., blood, swarms, and hail], G-d said to Moses, Rise up early in the morning … lo he cometh forth to the water. In line with the plain meaning of Scripture, to ease himself. Pharaoh claimed to be a god and asserted that [because of his divine powers], he did not need to ease himself. Therefore he would rise early and go forth to the Nile to ease himself in secret.” This interpretation of Rashi which is based upon a Midrash (Tanchuma Va’eira, 14) is alluded to by Ramban’s words that his own explanation is “in line with the plain meaning of Scripture,” thus implying that there is a Midrashic or homiletic interpretation of the verse, namely, that of Rashi mentioned above. this was the time when the kings were wont to go forth in the morning to enjoy themselves in the waters, and the Holy One, blessed be He, commanded Moses to go there. The reason for it is that since the plague of blood was the first one [of the plagues], He wanted Moses to do it in the sight of the king and without fear of him. This is the sense of the expression, and thou shalt place thyself towards him. Similarly, in the plague of swarms it is said, Rise up early in the morning, and stand before Pharaoh; lo, he cometh forth to the water, and also in the case of hail it is said, Rise up early in the morning and stand before Pharaoh, which likewise took place when the king went forth to the water. He wanted these two plagues to be wrought [at that particular time] because since swarms and hail brought death and punishment upon the people, the Holy One, blessed be He, wanted the warning to be given in the sight of all people. When the king went forth to the water, a multitude of people followed him, and when Moses will then forewarn him in their sight, perhaps they might plead with their master to return from his evil way. In case they do not do so, they will deserve punishment. But in the case of the other plagues, the warning given to the king alone was sufficient, and therefore in connection with them it is said, Go in unto Pharaoh, meaning that Moses was to go into the palace. In the case of gnats and boils, it does not say [that Moses was to go into the palace] because Aaron had to strike the dust of the earth [to bring on the plague of the gnats], and in the king’s palace there is no dust, as it has a pavement of green and white marble. In the case of boils, Moses had to throw the soot of the furnace heavenward. Thus [we must say that] these two plagues, [i.e., gnats and boils], were done in the sight of Pharaoh when he was in the court of the garden of the king’s palace, or some similar place.