I actually laughed at this example. Have you ever studied whale evolutionary history? It is honestly THE WORST example you could have given. They go from aquatic to terrestrial and back to aquatic in, if I recall correctly, 10 million years or so.
Meanwhile, we have fossil jellyfish from 500 million, horseshoe crabs from 440m, coelacanth from 400m, and crocodiles from 250m.... and yet, not much if any change?
You are the laughable one. Why would aquatic animals need to have vast changes when they have stayed in the water? Meanwhile the furry four legged packicetus would of course need drastic changes to adapt to water and eventually staying in there full time