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 7:1ENTRANCE INTO THE ARK. (Gen. 7:1-24)
And the Lord said unto Noah, Come thou and all thy house into the ark--The ark was finished; and Noah now, in the spirit of implicit faith, which had influenced his whole conduct, waited for directions from God.
gen 7:2Of every clean beast . . . fowls--Pairs of every species of animals, except the tenants of the deep, were to be taken for the preservation of their respective kinds. This was the general rule of admission, only with regard to those animals which are styled "clean," three pairs were to be taken, whether of beasts or birds; and the reason was that their rapid multiplication was a matter of the highest importance, when the earth should be renovated, for their utility either as articles of food or as employed in the service of man. But what was the use of the seventh? It was manifestly reserved for sacrifice; and so that both during Noah's residence in the ark, and after his return to dry land, provision was made for celebrating the rites of worship according to the religion of fallen man. He did not, like many, leave religion behind. He provided for it during his protracted voyage.
gen 7:4For yet seven days--A week for a world to repent! What a solemn pause! Did they laugh and ridicule his folly still? He whose eyes saw and whose heart felt the full amount of human iniquity and perverseness has told us of their reckless disregard (Luk 17:27).
gen 7:9There went in two and two--Doubtless they were led by a divine impulse. The number would not be so large as at first sight one is apt to imagine. It has been calculated that there are not more than three hundred distinct species of beasts and birds, the immense varieties in regard to form, size, and color being traceable to the influence of climate and other circumstances.
gen 7:16and the Lord shut him in--literally, "covered him round about." The "shutting him in" intimated that Noah had become the special object of divine care and protection, and that to those without the season of grace was over (Mat 25:10).
gen 7:17the waters increased, and bare up the ark--It seems to have been raised so gradually as to be scarcely perceptible to its occupants.
gen 7:20Fifteen cubits upward . . . and the mountains were covered--twenty-two and a half feet above the summits of the highest hills. The language is not consistent with the theory of a partial deluge.
gen 7:21all flesh died . . . fowl . . . cattle, and . . . creeping thing--It has been a uniform principle in the divine procedure, when judgments were abroad on the earth, to include every thing connected with the sinful objects of His wrath (Gen 19:25; Exo 9:6). Besides, now that the human race was reduced to one single family, it was necessary that the beasts should be proportionally diminished, otherwise by their numbers they would have acquired the ascendancy and overmastered the few that were to repeople the world. Thus goodness was mingled with severity; the Lord exercises judgment in wisdom and in wrath remembers mercy.
gen 7:24an hundred and fifty days--a period of five months. Though long before that every living creature must have been drowned, such a lengthened continuance of the flood was designed to manifest God's stern displeasure at sin and sinners. Think of Noah during such a crisis. We learn (Eze 14:14) that he was a man who lived and breathed habitually in an atmosphere of devotion; and having in the exercise of this high-toned faith made God his refuge, he did not fear "though the waters roared and were troubled; though the mountains shook with the swelling thereof" [Psa 46:3].