The locomotion for many modern robotic systems is optimized for a single target domain - aerial, surface or underwater. In this work, we address the challenge of developing a robotic system capable of controlled motion in air and underwater. Further, we explore the particular challenge of dynamic transitions between air and water. We propose Dipper, an aerial-aquatic hybrid vehicle. Dipper is a light-weight fixed-wing UAV with actively swept wings. The bio-inspired system is not only capable of maneuvering efficiently during flight and underwater, but can also perform dynamic aerial-aquatic transitions. We describe the design, construction and testing of the Dipper prototype, and demonstrate repeatability and robustness especially during the transition phases.