Why did go Pharaoh go each morning to the Nile?
Rashi: Pharaoh made out to be a god who did not need to relieve himself; so he would go early in the morning (before anybody was around) to do so.
Ramban (8:15) and Rashbam: He went for a morning stroll, as is the way of kings.
Targum Yonasan: He would go there regularly to practice witchcraft.
Why, for Makos Dam, Arov and Arbeh, did Hashem instruct Moshe to go and meet Pharaoh when he went to down to the Nile?
Ramban (later) 1 : For Makas Dam, because it was the first Makah, and Hashem wanted Moshe to confront him fearlessly as he made his way there in a show of strength. 2 And for Arov and Barad, which caused widespread death, he wanted to warn Pharaoh in front of the many people (who customarily would accompany the king on his stroll), to give them the opportunity to prevail upon Pharaoh to submit to Moshe's demands.
Da'as Zekenim, Hadar Zekenim (26): These were new warnings, i.e. they were not preceded by a Makah about which he was warned.
By the other plagues where it sufficed to warn Pharaoh privately in his palace, the Torah writes "Bo el Pharaoh (See Shemos 7:26, Shemos 9:1 & Shemos 10:1). Why does the Torah not use that expression in connection with the lice and the boils?
Ramban (later) 1 : Because regarding the former, Aharon had to strike the earth, and the floors of Pharaoh's palace were presumably made of marble; whereas regarding the latter, Moshe had to throw the soot up toward the sky, in which case, he would have had to meet Pharaoh outside in the garden or courtyard.