Waterlines

This is a far-future mystery story taking place on a wintry planet colonized by a few hundred descendents of the Earth diaspora, who share the planet by treaty with a race of humanoid-robot symbionts who live under the sea. When the aliens bring three human bodies to the colony administrator, he must figure out who they are (no one is missing) and how and why they were killed – but as he begins to investigate, he is targeted by the killers, who are hiding a very big secret.