Also changing from a land mammal to a aquatic mammal isn’t simply growing flippers and fins. (As if that is simple) It would also require a drastic metabolic change. No mammal could live in the water if it needed to resurface every 30-60 seconds. Also the animal would need to be able to cope with the rapid pressure changes and need a mechanism to prevent the bends from occurring.
In the video I mentioned David B first discusses the engineering challenge of changing a car to a submarine. This would obviously be a huge redesign. So now lets discuss the changes needed to make to a cow so that it can live in the open ocean. He discusses system after system that would have to change and said he quit counting at 50,000. So lets quantify that many unguided random changes. No one has ever even tried to figure out how many failed changes it would take before you get an actual profitable change. The time compression of the TOE forces all this to happen in some small number of millions of years so these changes would have to be happening constantly.
Reality shows us that animals have build in methods of correcting errors in DNA. Change just doesn't happen only cancer and birth defects that are killed off.
IIRC, they expire prior to deep dives, so less open cavity in the body to crush under depth. They dont breathe until they break the surface, and can oxygenate, then not exchange gas for a long time, (while under water)