Genesis 6:17

And I, behold, I bring a flood of waters upon the earth to swallow up all flesh which hath in it the spirit of life from under the heavens: whatever is upon the earth shall be swept away.

AND, BEHOLD, I DO BRING THE FLOOD. “Now I agree with those angels who long ago said to Me [when I was about to create man], What is man that Thou art mindful of him.” Thus the words of Rashi quoting Bereshith Rabbah.
But I wonder: in what way did He “agree” with them since He left man a remnant for a great deliverance through Noah and his sons and all living things to increase their seed as the sand! Perhaps this agreement with their opinion signified [His intent which was to come to fruition] when He would show them no mercy. [However, when the Divine quality of mercy was introduced, it resulted in the deliverance of mankind through Noah.]
By way of the truth [the mystic lore of the Cabala], va’ani (And I) is similar to: And I also will chastise you; (And I) thus intimates the attribute of judgment. so also, I am He! behold, My covenant is with thee; And as for Me, this is My covenant. The verse here thus states: “I also will have My hand on them for having destroyed the earth.” This is why He said [after the flood]: This is the token of the covenant which I make. — He maketh sore, and He bindeth up. The learned student [in the mystic lore of the Cabala] will understand.

ואני הנני מביא את המבול, G’d is saying, that Noach’s task is to build the ark, whereas He, when the time expires, will bring on the deluge He had threatened as soon as the ark would be completed. The word מבול is a variant of מפלה, referring to something lost as in Deut. 14,21 כל נבלה, a carcass being a life lost. The definitive article ה in המבול refers to the destruction, the nature of which had not previously been spelled out. [my paraphrasing. Ed.]

ואני...המבול מים, the word מבול after the word המבול is missing, a word which should have been written since we had not heard about a concept called מבול previously. There are some other parallels to this missing construct form when we would have expected it, as in Joshua 3,15 הארון הברית. The basic root of the word מבול is נבל, in the same sense as in Job 38,37 ונבלי מים מי ישכיב? “who can tilt the water bottles from the sky?” This is a reference to rain emanating from the celestial regions. At the same time the word also contains an allusion to “falling”, מפלה, as in Isaiah 1,30 נובלת עליה, things which fall from the sky and are poured on to earth The word מבול is an adjective describing a certain type of rain. The word גשם on the other hand, is a noun. This is why it is possible to use the word מבול concerning any matter that drops out of the sky, such as snow, fire, hail, etc. Also, this is the reason why the Torah did not need to write גשם מים מתחת השמים, if the Torah had not added the words כל אשר בארץ, I would have thought that the fish had also been meant to die. The word בארץ in our verse therefore means “on the dry land.” The decree of destruction did not include the fish because since they do not share man’s habitat they did not adopt man’s corrupt ways. (compare Zevachim 113)