I can't remember the title or author of a book I read as a child. It goes something like this

She is an investigator for a multi- planet/star system government organization. On a planet settled relatively recently someone has died/murdered with an important piece of information missing. The culture that has developed there is a bit difficult for them to penetrate so she is sent. It is a water planet and she is bioengineered to have gills and be able to handle the life there. She finds intrigue, maybe an undiscovered sentient species . . . .

Thanks for the help.