Many aquatic organisms use drag-based propulsion methods. At low Reynolds numbers (Re), animals stop moving as soon as their appendages do—they do not coast or glide. However, some animals swim at intermediate Re, where viscous forces are balanced with inertial forces. Here, gliding becomes possible as the animals grow larger and swim faster. This transition from low to intermediate Re, though bridged by many animals, is not well-understood. We investigated the swimming kinematics of three specific families of freshwater insects with similar drag-based swimming techniques: water boatmen (Corixidae), backswimmers (Notonectidae), and diving beetles (Dytiscidae). We collected synchronized high-speed video of the three-dimensional swimming trajectories of these insects across a range of Re, from less than 10mm to over 20mm in length. We compared swimming and turning characteristics across all three species and a range of Re. We found that the 3D trajectories displayed a spectrum of discrete vs. continuous behavior, indicative of the important role of both viscous and inertial forces in the intermediate Re regime. Our results provide important insight into organism-environment interactions at the millimeter-to-centimeter scale and into intermediate Re swimming, which is not as well understood as low Re and high Re swimming. In light of recent developments in the miniaturization of sensors, devices, and robots, our results may also provide a platform for bioinspired engineering at intermediate Re.