Cachalot

A guilt-ridden Earth had turned Cachalot over to the few surviving cetaceans as a perpetual refuge - a planet whose surface was one great ocean, where the remnants of the whales, porpoises and dolphins could pursue their lives and perhaps even the development of an intelligence even greater than man's.
Humans on Cachalot were strictly confined to a few islands and the floating towns, prospering from the wealth of its sea. The cetaceans seemed to have forgiven the thousands of years of terror and slaughter they had suffered - some had even befriended selected humans.
But something was destroying the towns of Cachalot - leaving no clues...or survivors.