PathFinder: TodHunter Moon, Book 1

People traveled to a distant location. Some people have gills. However, the only way to find out if you have them is to go into the water and "drown". If you find you can actually breathe, you have them. (One character near the end does.)

Septimus Heap and his family play roles here. The adventures are different than those in the previous series. The characters are now more mature, but they still have dragon adventures. (Why do I have such a hard time remembering the details?)