11 In the six hundredth year of Noah’s life, in the second month, on the seventeenth day of the month, on that day all the fountains of the great deep burst open, and the floodgates of the sky were opened.
Genesis 7:11
So, like, most of the water probably came from underground, not from the rain. Though I'd imagine both were pretty bad.
Not saying the story is true or anything. Just pointing out the straw man, since the Bible doesn't claim all the water was from rain.