Beneath the Surface - Lindsay Buroker

Beneath the Surface is the sixth novel in the Emperor's Edge series, but the author has designated it as 5.5. She wrote it so I'm not going to argue. It's a great book regardless.

Amaranthe's team gets separated trying to avoid Enforcers. The goal is to get back to the capital by steamboat, but there's something on the boat that threatens the entire empire. Amaranthe and her Emperor's Edge have to eliminate that threat. Another great story. Very highly recommended.