EntirelyPossible,
The ark was a delivery vessel.
Why did God bother to have the Ark built? Why did Noah and his family use tar to seal out the waters?
It is because God expects a measure of willingness from each person to work towards their own salvation.
Much probably happened during Noah and his family's stay on board the Ark. There were earthquakes and undersea volcanic eruptions. The result was the receding of the waters were not to their original boundaries.
Land masses under went change from caverns, rising land masses and trenches that were created from God's hand being involved. This altered the original water boundaries and created new ones.
1 Peter 3:18-22
New International Version (NIV)
18 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. 19 After being made alive, [a] he went and made proclamation to the imprisoned spirits— 20 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, 21 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. [b] It saves you by the resurrection of Jesus Christ, 22 who has gone into heaven and is at God’s right hand—with angels, authorities and powers in submission to him.