Genesis 6:1 - 8:22
ESV - 1 When man began to multiply on the face of the land and daughters were born to them. 2 The sons of God saw that the daughters of man were attractive. And they took as their wives any they chose.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Even if God wanted to spare the animals and He could do that if He wanted, think of how many caregivers would be required. On the ark he placed enough animals to continue their "being" under the care of Noah and his sons. God saved what was "good" and He did it by having man do the work. He could have created everything again but he chose to use man and not to kill all that He made that was good. What an awesome God we serve!
Land animals and humans had to be preserved somehow to continue their lineage. Birds also, because they have to have a place to perch and some way to find food. Neither could do any of that with the earth covered by water.
Fish and other marine life on the other hand, had some problems with a flooded environment, but could weather the storm, so to speak. They were right at home!