In the passage of VeHayah Im Shamo'ah, the second paragraph of the Shema prayer, Hashem tells us that if we listen to Him and keep his Mitzvot, we will be rewarded by receiving rain to grow our crops. However, says Hashem, if we don't keep his mitzvot (God forbid), we will be punished and not receive any rain. In Parashat Noach, though, where all the people sinned greatly, Hashem does not hold back the rain but overwhelms us with it by sending the Flood.
We see from here the power that water has on a person's life: how it can be used to destroy us, while it is vital to our basic survival. Logically, then, if we don't listen to Hashem, who is essential to our very being, we won't get any rain. When Hashem tells us to teach our children later in the aformentioned passage, He wants us to teach the story of Noach and how we can be rewarded and punished through the same resource. That is the same idea expressed by the rainbow. Hashem is giving Noach and future generations a promise that he won't destroy the whole world again, but he is also giving us a powerful reminder that as easily as we can go astray from following Hashem, Hashem can punish us by taking from us our essentials. May we all remember and take to heart the story of Noach in order to better our Avodat Hashem.