This book would be from the 1980s or earlier. I believe the main character was a woman. They are on another planet covered mostly or fully in water. Not entirely sure of the plot. Whales and dolphins could "talk" with people and helped them on this planet.
Dolphins were more child-like and the killer whales were more mature. The larger the whale the smarter and more independent they seemed to be.
At the end, if I recall correctly, one of the blue whales learns to take flight.
My answer is the novel Cachalot, by Alan Dean Foster.
The story is set on another world, which humans gave to the whales for colonization after their high intelligence was verified. The planet also had a small human population, who lived on floating cities. The novel's plot involved the search - humans cooperating with whales - for something unknown that was destroying the human habitations. This novel came out in 1980. The planet's name, Cachalot, is French for 'sperm whale'.
There were no land animals native to Cachalot. So there were no reptiles or mammals for true birds to evolve from. In the absence of true birds or flying snakes or their relatives, the ichthyorniths, with their water-carrying body sacs, had adapted to a partial aerial existence, spending as little time in the water as possible, breeding and living in a mostly predator-free niche left to them by a nonwasteful nature
