The water likely came from inside the earth. Genesis 7:11-12 In the six hundredth year of Noah’s life, on the seventeenth day of the second month—on that day all the springs of the great deep burst forth, and the floodgates of the heavens were opened. 12 And rain fell on the earth forty days and forty nights
There is also the indication that the earth was once surrounded by some sort of canopy. This would have limited the amount of ultraviolet light hitting the earth. In Genesis 7 there is also the indication that rather than extremely high mountains, there were large hills Genesis 7:17-20
17 And the flood was forty days upon the earth; and the waters increased, and bare up the ark, and it was lift up above the earth.
18 And the waters prevailed, and were increased greatly upon the earth; and the ark went upon the face of the waters. 19
And the waters prevailed exceedingly upon the earth; and all the high hills, that were under the whole heaven, were covered. 20 Fifteen cubits upward did the waters prevail; and the mountains were covered.
Again, rather than oceans, there was a shallower sea. Think in terms of something like the Great Lakes and the Mediterranean . SO, what may have happened was that the earth's crust was ruptured in some way ----- likely an asteroid or more. The water within the earth erupted and shot into the atmosphere and rained down upon the earth covering the earth over a 40 day period. The LORD eventually, brought a wind to dry the land. Genesis 8:1 But God remembered Noah and all the wild animals and the livestock that were with him in the ark, and he sent a wind over the earth, and the waters receded, and apparently began a revamping of the earth's surface, creating mountains and valleys.
Psalm 104: 1-9
Praise the Lord, my soul.
Lord my God, you are very great;
you are clothed with splendor and majesty.
2 The Lord wraps himself in light as with a garment;
he stretches out the heavens like a tent
3 and lays the beams of his upper chambers on their waters.
He makes the clouds his chariot
and rides on the wings of the wind.
4 He makes winds his messengers,
flames of fire his servants.
5 He set the earth on its foundations;
it can never be moved.
6 You covered it with the watery depths as with a garment;
the waters stood above the mountains.
7
But at your rebuke the waters fled,
at the sound of your thunder they took to flight;
8 they flowed over the mountains,
they went down into the valleys,
to the place you assigned for them.
9
You set a boundary they cannot cross;
never again will they cover the earth.
! Chronicles 1:19
19 Two sons were born to Eber: One was named Peleg, because in his time the earth was divided; his brother was named Joktan.
( "Peleg" means "division" or "dividing canal" in Hebrew)
So, a relatively flat earth with hills and some high hills. At some point valleys and basins were formed for where the water would drain and greater mountains pushed up. Most of the earth is covered with water --- approx. 70%. And the ocean has areas that are 7 miles deep. So in fact, the Flood Waters are still here. It is likely that the wind was the beginning of the Ice Age, and would have formed ice sheets and also would have removed water depth.