Moshe argues with Hashem (4:1) that he should not be sent to redeem the Jewish people because "hein lo ya'aminu li v'lo yishmi'u b'koli ki yomru lo nirah elecha Hashem", the Jewish people will not believe him, will not listen to him, and will say Hashem did not appear to him. The Netziv explains that Moshe did deny that the Jewish people would believe they were going to be redeemed, rather he felt that they would deny that he was chosen as the redeemer. After all, Moshe had been raised as an Egyptian prince in his childhood; immediatly after rescuing his fellow Jew he was forced to flee for his life to the wilderness and was not heard from for years; his brother was already established as a prophet. Why would anyone accept his credibility?
Moshe therefore was given three signs: 1) the staff changing to a snake and then back to a staff when its tail was grabbed; 2) Moshe's hand turned leperous and was healed; 3) the water drawn from the Nile changed to blood. Explains the Netziv: 1) although Moshe was on the bottom rung of the social ladder in Bnei Yisrael, much like the snake when grabbed by its tail, that tail transforms into the top of the staff, so too, Moshe would become the top of Hashem's staff; 2) much like the hand healed of leperousy is healthier than the hand before being afflicted, Moshe upon his second foray into Egypt would be stronger than the first time around; 3) and there are still those who refuse to accept him, just as the water of life would turn to blood, the events of geulah would bring death to those who deny the plan of Hashem.
The Redeemer may be the person we least expect, it may be a person who has a history of failure in the past who we are not willing to give a second chance to, it may be a person who has been off the radar screen and has long since vansihed from our list of heroes, but, as the Netziv writes, it is in our hands to see that the water does not become blood, that we stand ready to accept the geulah shlaimah with unquestioning emumah in the yad Hashem. (Editorial comment: In other words, don't second guess the yad Hashem. You may think redemption by a former Egyptian prince with a lisp who spent the past 80 years alone in the desert is a bad idea, or that establishing reishit tzimchat geulateinu through those far from ruchniyus is a bad idea, but you're not in charge of the overall plan).