Whales evolved from land-dwelling mammals. There is actually a very clear evolutionary chain from them, if you bother to do a Google search. Whales are evolving today, too, as the potential speciation amongst Orcas shows. Whales do not seem to be evolving into gill-breathers.
Genetic mutations in recorded history have never produced a useful mutation.
DNA was discovered in the 20th century. Throughout most of recorded history, people did not even know to look for genetic mutations.
When it can't defend itself, it generates abberations and never has been observed to generate one that was useful.
Ah, the "No benefical mutations argument." Paging jennyp.
The genetic mutation in feline leukemia virus that allowed this virus to infect dogs is an example of a beneficial mutation. (It seems to be dated to Canada in 1972. Blame Canada!)