Neptune's brood - Charles Stross

She was looking for her sister. She found Atlantis.

Krina Alizond is a metahuman in a universe where the last natural humans became extinct five thousand years ago. When her sister goes missing she embarks on a daring voyage across the star systems to find her, travelling to her last known location - the mysterious water-world of Shin-Tethys.

In a universe with no faster-than-light travel that's a dangerous journey, made all the more perilous by the arrival of an assassin on Krina's tail, by the 'privateers' chasing her sister's life insurance policy and by growing signs that the disappearance is linked to one of the biggest financial scams in the known universe.