A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
gen 6:2WICKEDNESS OF THE WORLD. (Gen. 6:1-22)
the sons of God saw the daughters of men--By the former is meant the family of Seth, who were professedly religious; by the latter, the descendants of apostate Cain. Mixed marriages between parties of opposite principles and practice were necessarily sources of extensive corruption. The women, religious themselves, would as wives and mothers exert an influence fatal to the existence of religion in their household, and consequently the people of that later age sank to the lowest depravity.
gen 6:3flesh--utterly, hopelessly debased.
And the Lord said, My spirit shall not always strive--Christ, as God, had by His Spirit inspiring Enoch, Noah, and perhaps other prophets (Pe1 3:20; Pe2 2:5; Jde 1:14), preached repentance to the antediluvians; but they were incorrigible.
yet his days shall be an hundred and twenty years--It is probable that the corruption of the world, which had now reached its height, had been long and gradually increasing, and this idea receives support from the long respite granted.
gen 6:4giants--The term in Hebrew implies not so much the idea of great stature as of reckless ferocity, impious and daring characters, who spread devastation and carnage far and wide.
gen 6:5God saw it . . . repented . . . grieved--God cannot change (Mal 3:6; Jam 1:17); but, by language suited to our nature and experience, He is described as about to alter His visible procedure towards mankind--from being merciful and long-suffering, He was about to show Himself a God of judgment; and, as that impious race had filled up the measure of their iniquities, He was about to introduce a terrible display of His justice (Ecc 8:11).
gen 6:8But Noah found grace in the eyes of the Lord--favor. What an awful state of things when only one man or one family of piety and virtue was now existing among the professed sons of God!
gen 6:9Noah . . . just . . . and perfect--not absolutely; for since the fall of Adam no man has been free from sin except Jesus Christ. But as living by faith he was just (Gal 3:2; Heb 11:7) and perfect--that is, sincere in his desire to do God's will.
gen 6:11the earth was filled with violence--In the absence of any well-regulated government it is easy to imagine what evils would arise. Men did what was right in their own eyes, and, having no fear of God, destruction and misery were in their ways.
gen 6:13And God said unto Noah--How startling must have been the announcement of the threatened destruction! There was no outward indication of it. The course of nature and experience seemed against the probability of its occurrence. The public opinion of mankind would ridicule it. The whole world would be ranged against him. Yet, persuaded the communication was from God, through faith (Heb 11:7), he set about preparing the means for preserving himself and family from the impending calamity.
gen 6:14Make thee an ark--ark, a hollow chest (Exo 2:3).
gopher wood--probably cypress, remarkable for its durability and abounding on the Armenian mountains.
rooms--cabins or small cells.
pitch it within and without--mineral pitch, asphalt, naphtha, or some bituminous substance, which, when smeared over and become hardened, would make it perfectly watertight.
gen 6:15And this is the fashion--According to the description, the ark was not a ship, but an immense house in form and structure like the houses in the East, designed not to sail, but only to float. Assuming the cubit to be 21.888 inches, the ark would be five hundred forty-seven feet long, ninety-one feet two inches wide, and forty-seven feet two inches high.
gen 6:16A window--probably a skylight, formed of some transparent substance unknown.
in a cubit shalt thou finish it above--a direction to raise the roof in the middle, seemingly to form a gentle slope for letting the water run off.
gen 6:17And, behold, I, even I, do bring a flood--The repetition of the announcement was to establish its certainty (Gen 41:32). Whatever opinion may be entertained as to the operation of natural laws and agencies in the deluge, it was brought on the world by God as a punishment for the enormous wickedness of its inhabitants.
gen 6:18But with thee will I establish my covenant--a special promise of deliverance, called a covenant, to convince him of the confidence to be reposed in it. The substance and terms of this covenant are related at Gen 6:19-21.
gen 6:22Thus did Noah--He began without delay to prepare the colossal fabric, and in every step of his progress faithfully followed the divine directions he had received.