Noah's flood was for a specific purpose... The Heavenly Father Genesis 6:6 And it repented the LORD that He had made man on the earth, and it grieved Him at His heart.
7. And the LORD said, “I will destroy man whom I have created from the face of the earth; both man, and beast, and the creeping thing, and the fowls of the air; for it repenteth Me that I have mad them.”
Then we are told why Noah and his family were elected to be saved on the ark to continue the blood line to the birth of Christ.
My understanding was that the water was both “locked up” in the heavens and beneath the crust of the earth. The surface of the planet was much “smoother” in that it didn’t have the high points and low points that it does now, post-flood, and the amount of surface water was much less.
During the flood, these waters were released to cover the relatively low land masses.
During this time of coverage and neo-post-flood, the earth’s surface changed dramatically and quickly, raising up land and lowering the oceans, so all this “excess” water is still here, but in the deepest recesses of the oceans.