What makes you think that the water needs to cover the whole earth?
For instance, the water may concentrate in one place first then another and another. Humans don't know all kinds of catastrophes to tell. What Noah saw is the place near him.
You really don’t have concept of sea level.
For it to cover the highest mountains, it would change the sea level for millennia. Much of the earth would still be underwater today, if it reach the height of Everest.
Even if the Genesis Flood covered much shorter mountains, like Ararat (Greater Ararat is currently over 5000 metres, Lesser Ararat, just 4000 metres), the Egypt and the Levant would still be completely covered today.
Sea level can change over periods of time, these changes would be over some tens of metres, not thousands of metres, which would be required to cover these high mountains.
Some have argue that the Flood was the result of the melting ice sheets from the last Ice Age, but that’s not true. There are lot of misconceptions about the ice age, and most of it come from creationist camp, attempting distort the evidences.
Unlike more ancient Ice Ages than the Quaternary Glaciation, the Quaternary Glaciation was as severe as previous glaciations, because only certainly regions of northern of Europe, Asia and North America were covered in ice sheets, but larger regions saw no ice sheets throughout the Quaternary Glaciation. There were pockets of ice sheets covering some mountain ranges (Alps, Caucasus, Hindu Kush, Himalayas, Andes, etc), but that’s only because of the high altitudes in these regions. Regions like the Italy (except the Alps), France, Spain, Hungary, the Balkan, Egypt, the Levant, Mesopotamia, etc, saw absolutely no ice sheets.
The last maximum extent of the Quaternary Glaciation, occurred 20,000 years ago. The amount of ice melted since then to the present, have only caused the net sea level to rise only 125 metres.
That rise is certainly not high enough to cover mountains let alone Ararat’s 2nd highest peak (Lesser Ararat).
And the Genesis Flood, supposedly took place between 2400 and 2100 BCE, depending on the estimates of years indicated in the Old Testament (eg Genesis genealogy, Exodus 12:40-41 and 1 Kings 6:1).
But the things the Levantine, Egyptian and Sumerian-Akkadian civilisations saw no such destruction as described in Genesis, and all archaeological evidences showed none of their cities swallowed by water, at the same time.
Byblos for instance, was a small town built in the coast 5000 BCE, and later became thriving and growing city since the start of Bronze Age in 3000 BCE, and remained inhabited today.
Not once has Byblos being destroyed and lay abandoned. At no point in time, have Byblos being destroyed in the 3rd millennium BCE.
If Everest, or even just Ararat were covered by the Flood, then Byblos would remain underwater today.
Sea levels don’t changed by kilometres, because that what it would take for Noah’s Ark to land on and disembark from Ararat.
Genesis clearly stated that it took 150 days to covered the highest mountains, and around another 150 days before Noah left the ark.
But as I stated about 125 metres rise, since the last maximum extent of glaciation, there are clearly not enough water for Genesis Flood to occur as it claim.
Where did all that water come from? Where did it disappear?
Creationists cannot answer that, except that God did it, is not really logical, nor realistic answers, and certainly not scientific ones.