Penetrate by Kathleen Kelly

The Amazon River can be a treacherous place even for the most experienced and hardened person. This is the lesson Kali, a ruthless pirate, learns after her murderous mission ends in disaster, and she's faced not only with her own death but that of the people on her ship. With her crew planning a mutiny and the leader of her criminal cartel breathing down Kali's neck, the life of Madeline Cross, an innocent scientist caught up in the fray, should be the least of her worries. Yet, the more Kali tries to ignore her, the more Maddie stresses Kali's shaky control. Killing her might be the only way Kali can survive. Maddie would be just one more life in a string of faceless murders. Except this life would have meaning; this life would matter.