The godwhale (1973)

Rorqual Maru was her name. She was a harvester - a vast plankton rake without a crop, abandoned by Earth society when the seas died. Part whale, part ship, and well over 600 feet long, she was left to rot in the sterile ocean. But suddenly, after centuries, the sea was no longer dead, and Rorqual Maru stirred from her slumber. She would set out once again to serve mankind. But mankind had forgotten all about Rorqual Maru...