The working principle and the capabilities of a new platform called the River Drifter are here presented. This technology has applications in the study of the hydrodynamics of coastal areas, rivers, and lakes. The River Drifter was designed for shallow water applications (1 m and deeper) to collect concurrent measurements of surface currents, three-dimensional velocity profiles underneath the device, water depth, and salinity. Here, we discuss how water level displacements can be inferred and used to measure the swell characteristics and to also correct the measured velocity. We also show how the local vorticity field can be computed. As an example application, we describe a study whose goal was to investigate the fate of a polluted river plume and how two River Drifters initially following the same path are characterized by very different final trajectories. The different behaviors of the two drifters are explained in terms of the local flow dynamics, which are strongly influenced by the seabed morphology, forcing the River Drifters to move in different directions.