1 Peter 3:20 is indeed a crucial verse, which appears in the following context (1 Peter 3:18-22):
For Christ also suffered once for sins, the righteous for the unrighteous, to bring you to God. He was put to death in the body but made alive in the Spirit. After being made alive, he went and made proclamation to the imprisoned spirits— to those who were disobedient long ago when God waited patiently in the days of Noah while the ark was being built. In it only a few people, eight in all, were saved through water, and this water symbolizes baptism that now saves you also—not the removal of dirt from the body but the pledge of a clear conscience toward God. It saves you by the resurrection of Jesus Christ, who has gone into heaven and is at God’s right hand—with angels, authorities and powers in submission to him.
To interpret this pericope it is important to keep in mind words of Jesus Christ like these: “Go into all the world and preach the gospel to all creation. Whoever believes and is baptized will be saved, but whoever does not believe will be condemned.” (Mark 16: 15-16; Matthew 28:19) „For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life.”(John 3:16). These quotations make it clear that Baptism is instituted by Jesus Christ to save all the world, where the term ‘world’ means “all creatures in need of Redemption”.
Thus, by stating that “the water in the Noah’s Flood symbolizes Baptism” Peter makes it clear that from “all creatures in need of Redemption” living at this time only “few people, eight in all” were saved.
Your interpretation seems to suggest that: 1) the 14 millions of Homo sapiens creatures living outside Mesopotamia were human beings endowed with free will, and therefore capable of being guilty of sin and in need of Redemption, nonetheless 2) the symbolism of Baptism does not apply to these creatures.
This would mean to suggest that Peter as divinely inspired author is denying the universal need of Baptism as means for reaching Salvation. So your interpretation could be tricky: it risks emptying the meaning of Baptism and Christ’s Cross, and thereby questioning the very foundation of Christian faith; and it risks also nurturing the idea that certain human beings today are irredeemable and not in Image of God.
The reason why the symbolism of Baptism doesn’t apply to the millions of Homo sapiens creatures living outside Mesopotamia at the time of Noah’s Flood is not that “their lands didn't flood” (notice that for Baptism you don’t need a flood, it is enough with a little spring or pond!). The reason is that these creatures were neither endowed with free will, nor capable of being guilty of sin, nor in need of Redemption.
However, these millions of Homo sapiens creatures living outside Mesopotamia were endowed by God with free will at the end of the Flood, and since this very moment (referred to in Genesis 9: 2-4) all Homo sapiens creatures on earth are in God’s image and in need of Redemption, and can reach Salvation through the Grace of Jesus Christ bestowed in Baptism.
Consequently, the Flood referred to in the Peter’s Epistles should not be called “global” but rather “universal” by analogy to the Last Judgment: Both these events concern all creatures in need of Redemption at the time they happen.
I am the first to endorse that Genesis 1-11, although referring to actual historical events, is not merely giving a historical report and aims primarily to teach us theological truth. However, when it comes to define this truth, one should be careful to neither contemn the data of science nor empty Christ’s Cross and Resurrection of their Salvation power.
If on believes that “God created humans in biological continuity with all life on earth, but also as spiritual beings” [BioLogos, What we believe, 10], one endorses actually that God created the human persons by endowing Homo sapiens creatures with free will so that they were capable to love Him but also to sin. Whether at 50,000 BC or 3,000 BC the number of these creatures was large. Thus one has to decide whether they were all endowed with free will at once or in a gradual process. On my part I think the process was gradual and still going on at the age of the Flood.
In support of this assumption I refer to the “sons of God” in Genesis 6: 2-4: These characters were precisely human beings created directly by God through endowing Homo sapiens creatures with free will; since their generation happened without mediation of any human parents’ will, they are properly called “sons of God”. And this emphasizes also that the Flood episode is an organization narrative to the aim of explaining how the New World emerges from the Ancient World, and “New Creation” happens.