Sub-aqueous granular flows are complex non-linear processes. In the ocean, the transport of granular materials at the bed depends on near-bed flow and turbulence as well as seafloor morphology and sediment properties. However, the feedbacks between these processes are still poorly understood in part due to the difficulty of making high resolution measurements without disturbing the near-bed flow. Acoustic remote-sensing technologies present a promising approach, being capable of achieving mm-scale range resolutions using MHz frequencies. However, to correctly interpret results, the interaction of MHz frequency sound with water-submerged sand-sized sediments must be understood. The first part of this presentation focuses on measurements of the geoacoustic properties of water-saturated sediment at high frequencies: sound speed and attenuation within the sediment and the reflection coefficient at the sediment-water interface. The second part focuses on sub-aqueous gravity-driven granular flows. The granular materials - 0.40 mm sand and 0.34 mm glass beads - were released from a cofferdam in a 15-cm wide inclined channel, producing O(1) cm thick granular layers moving at O(10) cm/s speeds. Vertical velocity profiles within the moving layer were measured using pulse-coherent Doppler profilers operating at MHz frequencies. Results are compared to velocity profiles measured by a side-view camera through the chute sidewall and surface velocity profiles across the chute measured by a top-view camera. Estimates of depth-averaged solids concentration are obtained from the best-fit parameters of a theoretical viscous flow model and compared to the measured concentration based on the attenuation of sound through the moving layer.