Through the Eye of a Needle — Hal Clement

Time was running out for Bob Kinnaird. Without much warning, the Hunter—the green protoplasmic alien that lived inside him and cured all his ills—had suddenly become his destroyer. Day by day Bob grew weaker and weaker, and only specialists from the Hunter''s distant world would know what was wrong with him and, more importantly, how to save him. But the only way searchers from his planet could find him was to locate his missing spaceship…a spaceship that had crashed beneath the ocean years before, its location still very much a mystery. Once again leading an investigation against time—as he had done so many years before—the Hunter knew he had to find comrades and find them fast…before someone murdered his best friend.