......(((The bible was written by man and the writing was kind of restricted to what mankind, at that time, could understand. all of the sealife did not die and the idea that water covered the entire planet to the top of mount Everest is nonsense))).....
The local flood concept is both ‘theologically’ and ‘scientifically’ unsound..... As near as I can tell, the only reason to hold to this concept is to gain the acceptance of secular scientists who deny God’s Word...... Once an individual starts down the compromise road, one error begets another, until all portions of Scripture which disagree with the secular viewpoint of the day are thrown out.
As for where did the waters go...Psalm 104 suggests an answer..... After the waters covered the mountains (verse 6), God rebuked them and they fled (verse 7); the mountains rose, the valleys sank down (verse 8) and God set a boundary so that they will never again cover the Earth (verse 9)[1]. They are the same waters!
Clearly, what the Bible is telling us is that God acted to alter the Earth’s topography. New continental landmasses bearing new mountain chains of folded rock strata were uplifted from below the globe-encircling waters that had eroded and leveled the pre-Flood topography, while large deep ocean basin were formed to receive and accommodate the Flood waters that then drained off the emerging continents.
Mountains and water
That is why the oceans are so deep, and why there are folded mountain ranges.
https://christiananswers.net/q-aig/aig-floodwater.html
Further...Jesus Christ based His teaching of coming judgment on all mankind on the fact that Noah’s Flood judged all mankind (Matthew 24:36-39; Luke 17:26-27)........ A local flood implies a ‘partial judgment’..... Likewise, Peter based his prophecy that the existing planet will ‘melt with fervent heat’ (II Peter 3:10)...... and an entire new heavens and new earth will be re-created (3:13) on the historical fact of the global Flood of Noah’s day (3:6).
All things considered, few doctrines are taught as clearly in Scripture as that of the global Flood.