Noah's flood was a river flood
Of the six surviving versions of the Ancient Near East flood story, the Epic of Atrahasis is the most explicit about the nature of the flood. According to Atrahasis III,iv,6-9 "Like dragonflies they [dead bodies] have filled the river. Like a raft they have moved to the edge [of the boat]. Like a raft they have moved to the riverbank."
Centuries later, an editor of the Epic of Gilgamesh XI, replaced the word "river" with the word "sea", thus turning the river flood into an ocean deluge. We can see the mythmaker's hand at work here. The editor copied the Atrahasis flood story word-for-word into the Epic of Gilgamesh, but the editor made several changes. He changed the line in Atrahasis "Like dragonflies they have filled the river" and substituted "Like the spawn of fish they fill the sea" in Gilgamesh XI.
The author(s) of Genesis did not describe Noah's flood as an ocean deluge. The words "sea" or ocean" do not appear in Genesis 6-8. Genesis 7:20 has the water rising only 15 cubits (22 feet): "Fifteen cubits upward rose the waters and covered the hills." If, after the ark grounded, Noah surveyed the flood damage and concluded that the flood was more than ten thousand cubits deep, why would he and the story tellers forget to mention such an impressive number while remembering to mention an insignificant number like 15 cubits? Fifteen cubits defines the magnitude of the flood. It was not a deluge, but it was deep enough to breach the levees and cover the lowlands and some hills with water for many miles in all directions.
There is no word meaning "deep" or "depth" or "draft" in the Hebrew text or the Greek text of Genesis 7:20. A literal translation from Hebrew is "Five ten cubits up-from-there rose the waters and they covered the hills." The word mi-la-ma'al-ah meaning 'up from there' modifies 'rose' in the 15 cubits clause and is not in the same clause as hills/mountains. It is a mistranslation to say the waters rose 15 cubits above the mountains. The 15 cubits refers to how much the water rose, not how deep the water was. Depths would be different at different locations. As a modern news reporter might say, the water rose 22 feet above flood stage.
Sumer was very flat. If the river rose 15 cubits (22 feet) and overflowed the levees, the river water would submerge the flood plain for many miles in all directions. If this covered all of the hills that Noah could see, then all these hills were less than 15 cubits high. They were not mountains; they were hills. This is further explained in chapter 4 of the Noah's Ark book.