Introduction
A careful parsing of Genesis 1:20-23 provides the following outline for Day 5:
1) There were fish, but they weren't teeming
2) There were birds, but they weren't filling the skies
3) There were no whales
4) There was an event that "let" sealife and birds flourish
5) Forms were produced according to their pre-existing kinds
6) Any land mammals in the whales' ancestry precede Day 6
A careful study of the KT Impact event provides the precise circumstances prescribed by the scriptures. This tells us that Day 5 occurred 65 Ma (million years ago),in association with the extinction of the dinosaurs.
Before the KT Impact
The dinosaurs went extinct aroung 65 Ma. By then, according to the PBS articles "The Evolution of Mammals" and "Birds Arrived Comparatively Late", the evolution of mammals, birds, and fish were well under way. According to the fossil record, the appearance of various life forms goes something like this:
dinosaurs began
Then 65 Ma, a meteor struck the Yucatán peninsula of Mexico. It wiped out the dinosaurs and most of the other life that had evolved up until that point. Most, but not all. As it turns out, the description derived above from Genesis 1:20-23 describes the situation there pretty well.
1) There were fish, but they weren't teeming
The seas were actually teeming with fish until 65 million years ago. However, they experienced massive losses at the same time the dinosarus died out. Almost right fterwards, they recovered brilliantly.
2) There were birds, but they weren't filling the skies
According to the article, "Birds Arrived Comparatively Late", "The ancestors of all today’s birds evolved later... between 65 and 53 million years ago, independently of the dinosaurs. This is the 'big bang theory' of birds." As for what birds evolved from, some think they evolved from dinosaurs. Others thing they evolved from reptiles. What they agree on is that birds were being kept in check by dinosaurs until 65 Ma. Without the dinosaurs to keep them in check, they flourished. In fact, their population recovered after fish, just as scripture says.
3) There were no whales
With fish flourishing 65 Ma, and birds flourishing 53 Ma, whales evolved around 45 Ma.
4) There was an event that "let" sealife and birds flourish
The extinction of the dinosaurs allowed sealife and birds to flourish.
5) Forms were produced according to their pre-existing kinds
Fish and birds evolved from their pre-existing kinds, as did whales.
6) Any land mammals in the whales' ancestry precede Day 6
The land animals that evolved on Day 6 resulted form the Grand Coupure, around 33 Ma... fully 12 million years after whales evolved on Day 5.
So Sealife and Birds Evolved from the Survivors of the Dinosaur Extinction Event, the KT Impact
The flourishing of sealife and birds, as a result of the KT Impact, reconciles a key reason why theologians and scientists cannot agree on Evolution. Old Earthers adopt Day Age theories that tell us Days 5 and 6 overlap, and that they overlap with the dinosaurs; Young Earthers correctly point out that such theories contradict the plain meaning of scripture... the Creation Days do not overlap. The key is that overlaying the science upon a literal interpretation of scripture explains everything.
The GeoCreationist perspective is:
1. Day 5 began with the destruction of most life on earth, 65 Ma
2. Fish recovered, according to their kinds, 65 Ma
3. Birds recovered by 53 Ma
4. Whales appeared 45 Ma
5. On Day 6, God let cattle, creeping things, and beasts, evolve according to their kind... 33 Ma
Summary and Conclusions
Scripture tells us that the seas and land produced the life we have, and that God removed any encumbrances to it doing so. Before Day 5, the land and the seas were evolving increasingly complex life, but when the KT Impact occurred, the seas were not teeming. So, God (i.e., Jesus, the master craftsman) stepped in. The earth recovered from its disaster, and the seas and air flourished with life.