John Calvin Commentary Genesis 7:16

"And they that went in, went in male and female of all flesh, as God commanded him: and Jehovah shut him in." — Genesis 7:16 (ASV)

And the Lord shut him in. This is not added in vain, nor should it be lightly passed over. That door must have been large, which could admit an elephant. And truly, no pitch would be sufficiently firm and tenacious, and no joint sufficiently solid, to prevent the immense force of the water from penetrating through its many seams, especially in an inrush so violent and a shock so severe.

Therefore, Moses, to prevent the vain speculations which our own curiosity would suggest, declares in one word, that the ark was made secure from the deluge, not by human ingenuity, but by divine miracle. Indeed, it is not to be doubted that Noah had been endowed with new ability and wisdom, so that nothing would be defective in the structure of the ark.

But so that even this favor would not be without success, it was necessary for something greater to be added. Therefore, so that we do not measure the method of preserving the ark by the limits of our own judgment, Moses teaches us that the waters were not prevented from breaking into the ark by pitch or bitumen alone, but rather by the secret power of God and by the intervention of His hand.