Some thoughts (all scriptures from NKJV):
The earth may have been watered by other than rain.
Gen.2 5-6 For the Lord God had not caused it to rain on the earth, and there was no man to till the ground; but a mist went up from the earth and watered the whole face of the ground.
Gen.2 10 Now a river went out of Eden to water the garden.
"Things not yet seen" is not required to, but could refer to, rain.
Heb.11 7 By faith Noah, being divinely warned of things not yet seen, moved with godly fear, prepared an ark for the saving of his household, by which he condemned the world and became heir of the righteousness which is according to faith.
The windows of heaven being open
seems to imply that it hadn't previously rained.
Gen.7 11 On that day all the fountains of the great deep were broken up, and the windows of heaven were opened.
Most telling of all, at least to me, is the strong implication of the text that the rainbow had not previously been seen.
Gen.9 13-17 “I set My rainbow in the cloud, and it shall be for the sign of the covenant between Me and the earth. It shall be, when I bring a cloud over the earth, that the rainbow shall be seen in the cloud; and I will remember My covenant which is between Me and you and every living creature of all flesh; the waters shall never again become a flood to destroy all flesh. The rainbow shall be in the cloud, and I will look on it to remember the everlasting covenant between God and every living creature of all flesh that is on the earth.” And God said to Noah, “This is the sign of the covenant which I have established between Me and all flesh that is on the earth.”
Though each of these may be interpreted differently, together they seem to allow the idea of no rain before the flood. Again, I am in no way adamant about it.
But if it is so, it is entirely by God's hand. Just as in a related text, it seems evident to me, though perhaps not to others, that after the flood, God changed the nature of animals:
Gen.9 2-3 “And the fear of you and the dread of you shall be on every beast of the earth, on every bird of the air, on all that move on the earth, and on all the fish of the sea. They are given into your hand. Every moving thing that lives shall be food for you. I have given you all things, even as the green herbs.”
This might also help us to understand why the animals so meekly entered the ark, because the fear of man was not yet upon them.
Interestingly, the fact that every moving thing was given for food, would tell us that the dietary restrictions of the law were not in place before Mount Sinai.
