I think the rain that caused the big flood was the first time it rained on earth. Somewhere in the Bible it says before the flood, moisture came from the earth--like a mist.
Yes, Gen 2:6 gives us the key clue as to the origin of our oceans and lightweight continents : a 20km to 50km deep layer of wet gravel that fell to the earth from the earth's primordial ring system of planetisimals 4.4 billion years ago. Our tidally captured moon perturbed this early ring system, about equal in mass, out of orbit and these baseball-sized, hydrate-rich chunks then were slowed down in the outer gas tendrils of earth's early atmosphere at perigee, slowly falling as a wet-gravel rain. This layer was then tectonically massaged for .5B years(Gen 2:6) resulting in distinct oceans and "iceberg" continents by 3.9B years ago. The planetisimals that hit the moon were FRIED, thus the same isotope ratios of earth-moon surfaces in the refractory elements. As to noah's flood, the only REAL number that survived the "fish-story" is 15 cubits. A cubit is the length from your elbow to your finger tips, about 18 inches. Thus 15 cubits = about 22 feet deep water. Mt Ararat is higher than 22 feet, yes? So, how to un-tangle this myth? Noah was a PAKISTANI farmer who lived in the delta region of the Indus River which floods irregularly(unlike the nile). He had been flooded out before, thus he built a 3 layer log RAFT, with tent, on a high point in his orchard, tethering it to a tree; so that he could ride out the next river-flood. Along comes a 4 day HURRICANE(cyclone in that part of the world)with a storm surge of 20 feet(the dirty NE quadrant). The 45 ft long, 8 ft wide, 4.5 ft high RAFT gets washed out to sea where the entire earth seems to be covered with WATER, to Noah the surface observer(curvature of the spherical earth). The raft moves around in the great CCW gyre-current of the arabian sea and back onto an outer sand bar and they have to wait for low tide to wade back to the true shore. Noah goes back to business, the RAFT rots away on a PAKISTANI beach 5500 years ago, NOT on Mt Ararat. 7 generations later his illiterate descendents, as goat/sheep herders, jazz up this old family story of sea-survival around a campfire in iraq, a FISH story that gets bigger with each telling... Another 7 generations go by, a 5 year old ABRAM is taught this myth as GOD'S TRUTH. You can convince a 5 year old of ANYTHING, a 95 year old of NOTHING...yes? And now, ala Paul Harvey, you know the rest of the story...