Vonda N McIntyre 
Superluminal [EPUB ebook] 

She gave up her heart quite willingly.After the operation…Laenea Trevelyan, a starship pilot, has her heart replaced with a machine so she can survive faster-than-light travel. Orca, a diver, divides her time between starships and the Strait of Georgia, where her relatives include a family of killer whales and a group of other divers, human beings who can exist underwater, and who are, technically, at war with the United States. Radu Dracul, a colonist from the alien world Twilight, having chosen to leave his home and become a starship crew member, discovers he has abilities he never dreamed of.