1967’s The Dolphins of Altair is an SF novel by Margaret St. Clair.
Fed up with brutal exploitation, the dolphins of Earth take a desperate step. They reach out telepathically to their oppressors. Three humans prove sympathetic: Secretary Madeline Paxton, dock worker Sven Erikson, a former soldier, and Navy psychiatrist Dr. Edward Lawrence.
The humans decide on direct action. Their first challenge; breaking captive dolphins out of the Half Moon Bay naval research station. A direct assault would be suicidal. A more oblique approach might work, one that might not suggest to the authorities that anything more than a natural disaster is to blame. The solution? Use a stolen mine to set off an earthquake of sufficient magnitude to breach Half Moon Bay’s defenses.
This provides only a brief respite. Nowhere on Earth is safe from the dolphin-exploiting humans. If they do not recapture the dolphins, they will very likely kill them outright. What can one handless race lacking all technology do against the cunning apes of the land?
It is fortunate that the dolphins can remember the long-ago days before humans and dolphins went their separate ways. In those days, there was a single, quasi-aquatic species that had mastered high technology that even modern humans cannot match. A species not native to earth, but to the Altair system. Humans and dolphins are kin descended from interstellar colonists.
What the dolphins cannot remember, they can learn from their distant cousins on Altair. What the dolphins cannot make themselves, their human allies can. To buy time for the dolphin race, the allies embark on a bold plan: melt the ice caps and flood humanity’s coastal regions.
Claiming that humans are descended from (comparatively) recently arrived extra-terrestrials is pretty silly. I suppose embellishing it by linking hominids and cetaceans is a lesser silliness, flying in the face of only a few million years of fossil evidence rather than the hundreds of millions of years of evidence that link both groups to the other terrestrial tetrapods. Odd that this book could have been published as late as 1967, when one would think that most SF readers would have rejected it as nonsense. But it sold, as did several other books of the time that alleged interstellar provenance for humans.
I should have thought that the publisher might also have balked at protagonists (the human-dolphin alliance) who are responsible for the deaths of billions of humans. Some died from the immediate effects of the sudden rise in sea levels; many more perished when all seacoast cities drowned, as did most of the infrastructure that makes modern civilization possible.
Not only was I appalled by the megadeaths, I was not charmed by writing (flat), plot (ad hoc), or characters (also flat). St. Clair could write good books; this is not one of them. However, it was short (which limited the suffering) and I did rather like the Lehr cover.
The Dolphins of Altair is available here (Amazon). It does not seem to be available from Chapters-Indigo.