Superluminal

To pilot an interstellar spaceship a very special kind of human being is required. In the strange superluminal world which ships enter beyond the speed of light, ordinary humans age and wither and die apparently in seconds. Would-be space pilots are modified surgically; their hearts are removed and replaced by mechanical devices. Popularly known as "aztecs", they live mostly apart from the population at large.

The story centres on three people: Laenea Trevelyan, a pilot; Radu Dracul, a scarred crewman from the colony world Twilight; and Orca, a woman who is a member of a community of divers — people surgically and genetically adapted for life under the oceans. Laenea and Radu are lovers, but their relationship seems doomed: Laenea's altered physiology makes it literally impossible for Radu to live in close proximity to her. But their destinies remain connected. During a routine mission Radu's ship becomes lost and he discovers that he can do what is hitherto been impossible, and survive superluminal transit without either surgery or drugs. After he returns home he discovers that Laenea's ship is missing, and, along with Orca and others, he joins a rescue mission.

Synopsis from Open Library.