Anne McCaffrey & Elizabeth Ann Scarborough 
Deluge 

In Deluge the selkie twins, Ronan and Murel, leave Petaybee on a mission to help rescue their friend Marmie, who has been falsely arrested on the orders of a corrupt Colonel. However, the Colonel has more power in the Company than they realized and they end up being imprisoned themselves and taken to the Gwinnet Incarceration Colony. There they have to try to evade the clutches of their old adversary Dr Mabu, an unscrupulous scientist who wants to study their unusual shape-changing ability, and doesn”t care how much pain her experiments cause them.
Meanwhile, the powerful and avaricious Company is making another attempt to take over the world of Petaybee for its resources, and the twins parents, Yana and Sean, along with the entire planet, must fight for the independence of their sentient world once and for all…